Catch Net::SSH::Disconnect when connecting to SSH

This error was experienced by @pearkes. It is thrown when the remote end
unexpectedly closes the remote end. This is usually caused by SSH not
being able to properly setup the connection.

lib/vagrant/communication/ssh.rb

@@ -153,6 +153,12 @@ module Vagrant
           # This happens if authentication failed. We wrap the error in our
           # own exception.
           raise Errors::SSHAuthenticationFailed
+        rescue Net::SSH::Disconnect
+          # This happens if the remote server unexpectedly closes the
+          # connection. This is usually raised when SSH is running on the
+          # other side but can't properly setup a connection. This is
+          # usually a server-side issue.
+          raise Errors::SSHDisconnected
         rescue Errno::ECONNREFUSED
           # This is raised if we failed to connect the max amount of times
           raise Errors::SSHConnectionRefused

lib/vagrant/errors.rb

@@ -333,6 +333,11 @@ module Vagrant
       error_key(:ssh_connection_timeout)
     end
 
+    class SSHDisconnected < VagrantError
+      status_code(83)
+      error_key(:ssh_disconnected)
+    end
+
     class SSHKeyBadPermissions < VagrantError
       status_code(12)
       error_key(:ssh_key_bad_permissions)

templates/locales/en.yml

@@ -131,6 +131,11 @@ en:
         means that the VM booted, but there are issues with the SSH configuration
         or network connectivity issues. Please try to `vagrant reload` or
         `vagrant up` again.
+      ssh_disconnected: |-
+        The SSH connection was unexpectedly closed by the remote end. This
+        usually indicates that SSH within the guest machine was unable to
+        properly start up. Please boot the VM in GUI mode to check whether
+        it is booting properly.
       ssh_key_bad_permissions: |-
         The private key to connect to this box via SSH has invalid permissions
         set on it. The permissions of the private key should be set to 0600, otherwise SSH will