Handle missing machine folder value

If the value of the machine folder can not be found in the
system properties, report a user friendly error message and
include relevant information in the logs for debugging.

lib/vagrant/errors.rb

@@ -1020,6 +1020,10 @@ module Vagrant
       error_key(:virtualbox_install_incomplete)
     end
 
+    class VirtualBoxMachineFolderNotFound < VagrantError
+      error_key(:virtualbox_machine_folder_not_found)
+    end
+
     class VirtualBoxNoName < VagrantError
       error_key(:virtualbox_no_name)
     end

plugins/providers/virtualbox/driver/version_5_0.rb

@@ -662,13 +662,17 @@ module VagrantPlugins
         end
 
         def read_machine_folder
-          execute("list", "systemproperties", retryable: true).split("\n").each do |line|
-            if line =~ /^Default machine folder:\s+(.+?)$/i
-              return $1.to_s
-            end
+          info = execute("list", "systemproperties", retryable: true)
+          info.each_line do |line|
+            match = line.match(/Default machine folder:\s+(?<folder>.+?)$/i)
+            next if match.nil?
+            return match[:folder]
           end
 
-          nil
+          @logger.warn("failed to determine machine folder from system properties")
+          @logger.debug("processed output for machine folder lookup:\n#{info}")
+
+          raise Vagrant::Errors::VirtualBoxMachineFolderNotFound
         end
 
         def read_network_interfaces

templates/locales/en.yml

@@ -1794,6 +1794,10 @@ en:
         VirtualBox is complaining that the installation is incomplete. Please
         run `VBoxManage --version` to see the error message which should contain
         instructions on how to fix this error.
+      virtualbox_machine_folder_not_found: |-
+        Vagrant failed to determine the machine folder on this host from
+        the VirtualBox system information. Try running the command again.
+        If this error persists, please report this error as a bug.
       virtualbox_mount_failed: |-
         Vagrant was unable to mount VirtualBox shared folders. This is usually
         because the filesystem "vboxsf" is not available. This filesystem is

test/unit/plugins/providers/virtualbox/driver/version_5_0_test.rb

@@ -186,4 +186,91 @@ OUTPUT
       expect(storage_controllers.first.attachments).to eq(attachments_result)
     end
   end
+
+  describe "#read_machine_folder" do
+    let(:system_properties) { VBOX_SYSTEM_PROPERTIES }
+    let(:machine_folder) { "/home/username/VirtualBox VMs"}
+
+    before do
+      allow(subject).to receive(:execute).
+                          with("list", "systemproperties", any_args).
+                          and_return(system_properties)
+    end
+
+    it "should read the default folder" do
+      expect(subject.read_machine_folder).to eq(machine_folder)
+    end
+
+    context "when default folder value is missing" do
+      let(:system_properties) { VBOX_SYSTEM_PROPERTIES.sub(/^Default machine folder:.+$/, "")}
+
+      it "should raise a custom error" do
+        expect {
+          subject.read_machine_folder
+        }.to raise_error(Vagrant::Errors::VirtualBoxMachineFolderNotFound)
+      end
+    end
+  end
 end
+
+VBOX_SYSTEM_PROPERTIES=%(
+API version:                     7_0
+Minimum guest RAM size:          4 Megabytes
+Maximum guest RAM size:          2097152 Megabytes
+Minimum video RAM size:          0 Megabytes
+Maximum video RAM size:          256 Megabytes
+Maximum guest monitor count:     64
+Minimum guest CPU count:         1
+Maximum guest CPU count:         64
+Virtual disk limit (info):       2199022206976 Bytes
+Maximum Serial Port count:       4
+Maximum Parallel Port count:     2
+Maximum Boot Position:           4
+Maximum PIIX3 Network Adapter count:   8
+Maximum ICH9 Network Adapter count:   36
+Maximum PIIX3 IDE Controllers:   1
+Maximum ICH9 IDE Controllers:    1
+Maximum IDE Port count:          2
+Maximum Devices per IDE Port:    2
+Maximum PIIX3 SATA Controllers:  1
+Maximum ICH9 SATA Controllers:   8
+Maximum SATA Port count:         30
+Maximum Devices per SATA Port:   1
+Maximum PIIX3 SCSI Controllers:  1
+Maximum ICH9 SCSI Controllers:   8
+Maximum SCSI Port count:         16
+Maximum Devices per SCSI Port:   1
+Maximum SAS PIIX3 Controllers:   1
+Maximum SAS ICH9 Controllers:    8
+Maximum SAS Port count:          255
+Maximum Devices per SAS Port:    1
+Maximum NVMe PIIX3 Controllers:  1
+Maximum NVMe ICH9 Controllers:   8
+Maximum NVMe Port count:         255
+Maximum Devices per NVMe Port:   1
+Maximum virtio-scsi PIIX3 Controllers:  1
+Maximum virtio-scsi ICH9 Controllers:   8
+Maximum virtio-scsi Port count:         256
+Maximum Devices per virtio-scsi Port:   1
+Maximum PIIX3 Floppy Controllers:1
+Maximum ICH9 Floppy Controllers: 1
+Maximum Floppy Port count:       1
+Maximum Devices per Floppy Port: 2
+Default machine folder:          /home/username/VirtualBox VMs
+Raw-mode Supported:              no
+Exclusive HW virtualization use: on
+Default hard disk format:        VDI
+VRDE auth library:               VBoxAuth
+Webservice auth. library:        VBoxAuth
+Remote desktop ExtPack:
+VM encryption ExtPack:
+Log history count:               3
+Default frontend:
+Default audio driver:            ALSA
+Autostart database path:
+Default Guest Additions ISO:     /usr/share/virtualbox/VBoxGuestAdditions.iso
+Logging Level:                   all
+Proxy Mode:                      System
+Proxy URL:
+User language:                   en_US
+)