Add error_code attribute to ChangeError

Also adds an additional `payload` keyword argument to the
`BaseService#error` method, which adds additional attributes to the
`result` Hash.

app/services/base_service.rb

@@ -50,16 +50,24 @@ class BaseService
 
   private
 
-  def error(message, http_status = nil)
+  # Return a Hash with an `error` status
+  #
+  # message     - Error message to include in the Hash
+  # http_status - Optional HTTP status code override (default: nil)
+  # pass_back   - Additional attributes to be included in the resulting Hash
+  def error(message, http_status = nil, pass_back: {})
     result = {
       message: message,
       status: :error
-    }
+    }.reverse_merge(pass_back)
 
     result[:http_status] = http_status if http_status
     result
   end
 
+  # Return a Hash with a `success` status
+  #
+  # pass_back - Additional attributes to be included in the resulting Hash
   def success(pass_back = {})
     pass_back[:status] = :success
     pass_back

app/services/commits/change_service.rb

@@ -30,7 +30,8 @@ module Commits
       error_msg = "Sorry, we cannot #{act} this #{type} automatically. " \
         "This #{type} may already have been #{act}ed, or a more recent " \
         "commit may have updated some of its content."
-      raise ChangeError, error_msg
+
+      raise ChangeError.new(error_msg, ex.error_code)
     end
   end
 end

app/services/commits/create_service.rb

@@ -3,7 +3,15 @@
 module Commits
   class CreateService < ::BaseService
     ValidationError = Class.new(StandardError)
-    ChangeError = Class.new(StandardError)
+    class ChangeError < StandardError
+      attr_reader :error_code
+
+      def initialize(message, error_code = nil)
+        super(message)
+
+        @error_code = error_code
+      end
+    end
 
     def initialize(*args)
       super
@@ -21,8 +29,9 @@ module Commits
       new_commit = create_commit!
 
       success(result: new_commit)
+    rescue ChangeError => ex
+      error(ex.message, pass_back: { error_code: ex.error_code })
     rescue ValidationError,
-           ChangeError,
            Gitlab::Git::Index::IndexError,
            Gitlab::Git::CommitError,
            Gitlab::Git::PreReceiveError,