communicators/ssh: nicer errors for SSH exceptions [GH-4367]

lib/vagrant/errors.rb

@@ -428,6 +428,10 @@ module Vagrant
       error_key(:multi_vm_target_required)
     end
 
+    class NetSSHException < VagrantError
+      error_key(:net_ssh_exception)
+    end
+
     class NetworkCollision < VagrantError
       error_key(:collides, "vagrant.actions.vm.host_only_network")
     end

plugins/communicators/ssh/communicator.rb

@@ -390,6 +390,9 @@ module VagrantPlugins
         rescue Errno::EHOSTUNREACH
           # This is raised if we can't work out how to route traffic.
           raise Vagrant::Errors::SSHNoRoute
+        rescue Net::SSH::Exception => e
+          # This is an internal error in Net::SSH
+          raise Vagrant::Errors::NetSSHException, message: e.message
         rescue NotImplementedError
           # This is raised if a private key type that Net-SSH doesn't support
           # is used. Show a nicer error.

templates/locales/en.yml

@@ -799,6 +799,15 @@ en:
         the proper location.
       multi_vm_target_required: |-
         This command requires a specific VM name to target in a multi-VM environment.
+      net_ssh_exception: |-
+        An error occurred in the underlying SSH library that Vagrant uses.
+        The error message is shown below. In many cases, errors from this
+        library are caused by ssh-agent issues. Try disabling your SSH
+        agent or removing some keys and try again.
+
+        If the problem persists, please report a bug to the net-ssh project.
+
+        %{message}
       nfs_bad_exports: |-
         NFS is reporting that your exports file is invalid. Vagrant does
         this check before making any changes to the file. Please correct