api: return error cause on parse error

Currently, when a query can't be parsed, the error is:
- logged to Sentry (which is useless to us),
- returned as a generic 'Internal Server Error' (which is useless to the
  user who made the query).

With this commit, the error is instead ignored from our logs (because it
is a user error), but the parse error details are returned to the user,
with the following format:

> {'errors': [{'message': 'Parse error on ")" (RPAREN) at [3, 23]'}]}

app/controllers/api/v2/graphql_controller.rb

@@ -8,6 +8,8 @@ class API::V2::GraphqlController < API::V2::BaseController
       operation_name: params[:operationName])
 
     render json: result
+  rescue GraphQL::ParseError => exception
+    handle_parse_error(exception)
   rescue => exception
     if Rails.env.production?
       handle_error_in_production(exception)
@@ -88,6 +90,15 @@ class API::V2::GraphqlController < API::V2::BaseController
     end
   end
 
+  def handle_parse_error(exception)
+    render json: {
+      errors: [
+        { message: exception.message }
+      ],
+      data: nil
+    }, status: 400
+  end
+
   def handle_error_in_development(exception)
     logger.error exception.message
     logger.error exception.backtrace.join("\n")