Specify default error translation key

I ran into this while working on my custom token generation.

- I accidentally triggered a `NoMethodError` which was rescued and
  wrapped here:
  https://github.com/doorkeeper-gem/doorkeeper/blob/master/lib/doorkeeper/models/access_token_mixin.rb#L137
- This was then fed into
  https://github.com/doorkeeper-gem/doorkeeper/blob/master/app/controllers/doorkeeper/tokens_controller.rb#L9
  and I got an error message from I18n about a missing translation key.

This changeset adds a default to prevent that from happening.

[fixes doorkeeper-gem#747]

lib/doorkeeper/oauth/error.rb

@@ -2,7 +2,11 @@ module Doorkeeper
   module OAuth
     class Error < Struct.new(:name, :state)
       def description
-        I18n.translate name, scope: [:doorkeeper, :errors, :messages]
+        I18n.translate(
+          name,
+          scope: [:doorkeeper, :errors, :messages],
+          default: :server_error
+        )
       end
     end
   end

spec/lib/oauth/error_spec.rb

@@ -4,15 +4,19 @@
 
 module Doorkeeper::OAuth
   describe Error do
-    subject { Error.new(:some_error, :some_state) }
+    subject(:error) { Error.new(:some_error, :some_state) }
 
     it { expect(subject).to respond_to(:name) }
     it { expect(subject).to respond_to(:state) }
 
     describe :description do
       it 'is translated from translation messages' do
-        expect(I18n).to receive(:translate).with(:some_error, scope: [:doorkeeper, :errors, :messages])
-        subject.description
+        expect(I18n).to receive(:translate).with(
+          :some_error,
+          scope: [:doorkeeper, :errors, :messages],
+          default: :server_error
+        )
+        error.description
       end
     end
   end