Improves the graphql_helpers used in GraphQL tests

This improves the graphql test helpers, failing earlier and more
explicitly.

spec/support/helpers/graphql_helpers.rb

@@ -3,6 +3,8 @@
 module GraphqlHelpers
   MutationDefinition = Struct.new(:query, :variables)
 
+  NoData = Class.new(StandardError)
+
   # makes an underscored string look like a fieldname
   # "merge_request" => "mergeRequest"
   def self.fieldnamerize(underscored_field_name)
@@ -158,8 +160,9 @@ module GraphqlHelpers
     post_graphql(mutation.query, current_user: current_user, variables: mutation.variables)
   end
 
+  # Raises an error if no data is found
   def graphql_data
-    json_response['data']
+    json_response['data'] || (raise NoData, graphql_errors)
   end
 
   def graphql_errors
@@ -173,8 +176,9 @@ module GraphqlHelpers
     end
   end
 
+  # Raises an error if no response is found
   def graphql_mutation_response(mutation_name)
-    graphql_data[GraphqlHelpers.fieldnamerize(mutation_name)]
+    graphql_data.fetch(GraphqlHelpers.fieldnamerize(mutation_name))
   end
 
   def nested_fields?(field)