Show server error messages instead of doing a separate number index

Currently in most JavaScript code I implemented new error codes are used to show messages to the user. Torsten and me agreed that instead of numbers the server should return the error messages which are shown directly to the user.

When a request contains multiple actions, like multiple apps or files to add, I suggest to collect all error messages and return them in a JSON array.

Related to #109 (closed)

mentioned in merge request !120 (merged)

closed via merge request !120 (merged)