Use GraphQL errors_on_object for collecting errors

Around 50% of our mutations were calling `object.errors.full_messages`
within their `#resolve` method, while others were using the
`#errors_on_object` helper in `BaseMutation`.

This MR changes all mutations to use `#errors_on_object`, and tweaks
the GraphQL API developer docs to mention the helper.

app/graphql/mutations/base_mutation.rb

@@ -9,7 +9,7 @@ module Mutations
 
     field :errors, [GraphQL::STRING_TYPE],
           null: false,
-          description: "Errors encountered during execution of the mutation."
+          description: 'Errors encountered during execution of the mutation.'
 
     def current_user
       context[:current_user]

app/graphql/mutations/issues/set_confidential.rb

@@ -19,7 +19,7 @@ module Mutations
 
         {
           issue: issue,
-          errors: issue.errors.full_messages
+          errors: errors_on_object(issue)
         }
       end
     end

app/graphql/mutations/issues/set_due_date.rb

@@ -19,7 +19,7 @@ module Mutations
 
         {
           issue: issue,
-          errors: issue.errors.full_messages
+          errors: errors_on_object(issue)
         }
       end
     end

app/graphql/mutations/issues/update.rb

@@ -33,7 +33,7 @@ module Mutations
 
         {
           issue: issue,
-          errors: issue.errors.full_messages
+          errors: errors_on_object(issue)
         }
       end
     end

app/graphql/mutations/merge_requests/set_assignees.rb

@@ -40,7 +40,7 @@ module Mutations
 
         {
           merge_request: merge_request,
-          errors: merge_request.errors.full_messages
+          errors: errors_on_object(merge_request)
         }
       end
     end

app/graphql/mutations/merge_requests/set_labels.rb

@@ -41,7 +41,7 @@ module Mutations
 
         {
           merge_request: merge_request,
-          errors: merge_request.errors.full_messages
+          errors: errors_on_object(merge_request)
         }
       end
 

app/graphql/mutations/merge_requests/set_locked.rb

@@ -21,7 +21,7 @@ module Mutations
 
         {
           merge_request: merge_request,
-          errors: merge_request.errors.full_messages
+          errors: errors_on_object(merge_request)
         }
       end
     end

app/graphql/mutations/merge_requests/set_milestone.rb

@@ -22,7 +22,7 @@ module Mutations
 
         {
           merge_request: merge_request,
-          errors: merge_request.errors.full_messages
+          errors: errors_on_object(merge_request)
         }
       end
     end

app/graphql/mutations/merge_requests/set_subscription.rb

@@ -18,7 +18,7 @@ module Mutations
 
         {
           merge_request: merge_request,
-          errors: merge_request.errors.full_messages
+          errors: errors_on_object(merge_request)
         }
       end
     end

app/graphql/mutations/merge_requests/set_wip.rb

@@ -21,7 +21,7 @@ module Mutations
 
         {
           merge_request: merge_request,
-          errors: merge_request.errors.full_messages
+          errors: errors_on_object(merge_request)
         }
       end
 

doc/development/api_graphql_styleguide.md

@@ -690,8 +690,9 @@ should look like this:
   # The merge request modified, this will be wrapped in the type
   # defined on the field
   merge_request: merge_request,
-  # An array if strings if the mutation failed after authorization
-  errors: merge_request.errors.full_messages
+  # An array of strings if the mutation failed after authorization.
+  # The `errors_on_object` helper collects `errors.full_messages`
+  errors: errors_on_object(merge_request)
 }
 ```