core: error if vagrant ssh -c with password

lib/vagrant/action/builtin/ssh_run.rb

@@ -26,11 +26,15 @@ module Vagrant
           # not yet ready for SSH, so we raise this exception.
           raise Errors::SSHNotReady if info.nil?
 
-          if info[:private_key_path]
+          info[:private_key_path] ||= []
+
           # Check SSH key permissions
           info[:private_key_path].each do |path|
             SSH.check_key_permissions(Pathname.new(path))
           end
+
+          if info[:private_key_path].empty?
+            raise Errors::SSHRunRequiresKeys
           end
 
           # Get the command and wrap it in a login shell

lib/vagrant/errors.rb

@@ -532,6 +532,10 @@ module Vagrant
       error_key(:ssh_port_not_detected)
     end
 
+    class SSHRunRequiresKeys < VagrantError
+      error_key(:ssh_run_requires_keys)
+    end
+
     class SSHUnavailable < VagrantError
       error_key(:ssh_unavailable)
     end

templates/locales/en.yml

@@ -650,6 +650,15 @@ en:
 
         Please make sure that you have a forwarded port that goes to the configured
         guest port value, or specify an explicit SSH port with `config.ssh.port`.
+      ssh_run_requires_keys: |-
+        Using `vagrant ssh -c` requires key-based SSH authentication, but your
+        Vagrant environmet is configured to use only password-based authentication.
+        Please configure your Vagrantfile with a private key to use this
+        feature.
+
+        Note that Vagrant can automatically insert a keypair and use that
+        keypair for you. Just set `config.ssh.insert_key = true` in your
+        Vagrantfile.
       ssh_unavailable: "`ssh` binary could not be found. Is an SSH client installed?"
       ssh_unavailable_windows: |-
         `ssh` executable not found in any directories in the %PATH% variable. Is an