core: refactor error initialization logic

/cc @tmatilai - This includes a test case for the symbol stuff, should
be okay now.

lib/vagrant/errors.rb

@@ -59,11 +59,14 @@ module Vagrant
         define_method(:error_namespace) { namespace }
       end
 
-      def initialize(message=nil, *args)
+      def initialize(*args)
+        key     = args.shift if args.first.is_a?(Symbol)
+        message = args.shift if args.first.is_a?(Hash)
         message ||= {}
-        @extra_data = message.dup
-        message = { :_key => message } if message && !message.is_a?(Hash)
-        message = { :_key => error_key, :_namespace => error_namespace }.merge(message || {})
+        @extra_data    = message.dup
+        message[:_key] ||= error_key
+        message[:_namespace] ||= error_namespace
+        message[:_key] = key if key
 
         if message[:_key]
           message = translate_error(message)
@@ -71,7 +74,7 @@ module Vagrant
           message = error_message
         end
 
-        super
+        super(message)
       end
 
       # The error message for this error. This is used if no error_key

templates/locales/en.yml

@@ -708,7 +708,6 @@ en:
         The synced folder type '%{type}' is reporting as unusable for
         your current setup. Please verify you have all the proper
         prerequisites for using this shared folder type and try again.
-      test_key: "test value"
       ui_expects_tty: |-
         Vagrant is attempting to interface with the UI in a way that requires
         a TTY. Most actions in Vagrant that require a TTY have configuration
@@ -816,6 +815,7 @@ en:
       vm_not_running: |-
         VM must be running to open SSH connection. Run `vagrant up`
         to start the virtual machine.
+      test_key: "test value"
 
 #-------------------------------------------------------------------------------
 # Translations for config validation errors

test/unit/vagrant/errors_test.rb

@@ -43,5 +43,11 @@ describe Vagrant::Errors::VagrantError do
       expect(subject.extra_data).to have_key(:data)
       expect(subject.extra_data[:data]).to eql("yep")
     end
+
+    it "should use a symbol initializer as a key" do
+      subject = klass.new(:test_key)
+      expect(subject.extra_data).to be_empty
+      expect(subject.to_s).to eql("test value")
+    end
   end
 end