[bugfix] Catch json syntax errors in the frontend + display a more helpful message (#804)

* catch json syntax errors

* update error msg

* re-throw error with cause

Co-authored-by: f0x52 <f0x@cthu.lu>

web/source/panels/lib/oauth.js

@@ -184,6 +184,12 @@ module.exports = function oauthClient(config, initState) {
 			} else {
 				return json;
 			}
+		}).catch(e => {
+			if (e instanceof SyntaxError) {
+				throw new Error("Error: The GtS API returned a non-json error. This usually means a network problem, or an issue with your instance's reverse proxy configuration.", {cause: e});
+			} else {
+				throw e;
+			}
 		});
 	}