Updated yaml parser error to be InvalidUsageError

- a yaml parser error can only be fixed by the user uploading a file, therefore it should be a 4xx, not a 5xx error
- an amp parser error indicates the amperize module is unable to handle a genuine case, and needs to be fixed at the code level

core/frontend/apps/amp/lib/helpers/amp_content.js

@@ -133,14 +133,16 @@ function getAmperizeHTML(html, post) {
 
                 if (err) {
                     if (err.src) {
+                        // This is a valid 500 GhostError because it means the amperize parser is unable to handle some Ghost HTML.
                         logging.error(new errors.GhostError({
                             message: `AMP HTML couldn't get parsed: ${err.src}`,
+                            code: 'AMP_PARSER_ERROR',
                             err: err,
                             context: post.url,
                             help: i18n.t('errors.apps.appWillNotBeLoaded.help')
                         }));
                     } else {
-                        logging.error(new errors.GhostError({err}));
+                        logging.error(new errors.GhostError({err, code: 'AMP_PARSER_ERROR'}));
                     }
 
                     // save it in cache to prevent multiple calls to Amperize until

core/frontend/services/settings/yaml-parser.js

@@ -18,8 +18,10 @@ module.exports = function parseYaml(file, fileName) {
     } catch (error) {
         // CASE: parsing failed, `js-yaml` tells us exactly what and where in the
         // `reason` property as well as in the message.
-        throw new common.errors.GhostError({
+        // As the file uploaded is invalid, the person uploading must fix this - it's a 4xx error
+        throw new common.errors.IncorrectUsageError({
             message: common.i18n.t('errors.services.settings.yaml.error', {file: fileName, context: error.reason}),
+            code: 'YAML_PARSER_ERROR',
             context: error.message,
             err: error,
             help: common.i18n.t('errors.services.settings.yaml.help', {file: fileName})