Move error name to BaseResponseError class

The "friendly" name of the error that's sent as a query param (?error=friendly_name)
on error responses is, by default, the name of the error class, formatted to snake case.

This change moves this definition to a method in the base class
`BaseResponseError#name_for_response`

This way, if ever needed, the `name_for_response` can be overridden by doing:
```rb
SomeName = Class.new(BaseResponseError) do
  def self.name_for_response
    :some_other_name
  end
end
```

lib/doorkeeper/errors.rb

@@ -39,14 +39,16 @@ class BaseResponseError < DoorkeeperError
       def initialize(response)
         @response = response
       end
+
+      def self.name_for_response
+        self.name.demodulize.underscore.to_sym
+      end
     end
 
     UnableToGenerateToken = Class.new(DoorkeeperError)
     TokenGeneratorNotFound = Class.new(DoorkeeperError)
     NoOrmCleaner = Class.new(DoorkeeperError)
 
-    ServerError = Class.new(BaseResponseError)
-
     InvalidRequest = Class.new(BaseResponseError)
     InvalidToken = Class.new(BaseResponseError)
     InvalidClient = Class.new(BaseResponseError)
@@ -60,6 +62,7 @@ def initialize(response)
     UnsupportedResponseMode = Class.new(BaseResponseError)
 
     AccessDenied = Class.new(BaseResponseError)
+    ServerError = Class.new(BaseResponseError)
 
     TokenExpired = Class.new(InvalidToken)
     TokenRevoked = Class.new(InvalidToken)

lib/doorkeeper/oauth/error_response.rb

@@ -10,8 +10,8 @@ class ErrorResponse < BaseResponse
       def self.from_request(request, attributes = {})
         new(
           attributes.merge(
-            name: request.error.to_s.demodulize.underscore.to_sym,
-            exception_class: request.error <= Errors::BaseResponseError ? request.error : nil,
+            name: request.error&.name_for_response,
+            exception_class: request.error,
             state: request.try(:state),
             redirect_uri: request.try(:redirect_uri),
           ),

spec/controllers/authorizations_controller_spec.rb

@@ -1181,7 +1181,7 @@ def query_params
         default_scopes_exist :public
       end
 
-      it "does not redirect" do
+      it "raises InvalidScope error" do
         expect do
           get :new, params: {
             client_id: client.uid,
@@ -1190,7 +1190,7 @@ def query_params
             redirect_uri: client.redirect_uri,
             state: "return-this",
           }
-        end.to raise_error(Doorkeeper::Errors::BaseResponseError)
+        end.to raise_error(Doorkeeper::Errors::InvalidScope)
       end
     end
 
@@ -1199,14 +1199,14 @@ def query_params
         default_scopes_exist :public
       end
 
-      it "does not redirect" do
+      it "raises InvalidRedirectUri error" do
         expect do
           get :new, params: {
             client_id: client.uid,
             response_type: "token",
             redirect_uri: "invalid",
           }
-        end.to raise_error(Doorkeeper::Errors::BaseResponseError)
+        end.to raise_error(Doorkeeper::Errors::InvalidRedirectUri)
       end
     end
   end