[FEATURE] Improve error handling in config UI

[R0Wi]

Related issues

#215
#183

Description

If the user opens up the workflow config UI, we query some data in the background which represents the current config state (for example the installed OCR languages which we show later in a dropdown menu). There is currently not really an error handling in case any of the API endpoints returns a statuscode != 200.

To do

We should improve our ajax service classes in https://github.com/R0Wi-DEV/workflow_ocr/tree/be65cde83a867b15e86c6c9594da8fa877c33089/src/service to handle any error responses. Our server implementations will return something like this in case of an unhandled 500 server error:

{ "error": "<MESSAGE>" }

Implementation plan

• Inject a possibility to show UI notifications into our ajax service classes (something like OC.Notification.showTemporary but ideally non-static)
• Implement error handling in case server sends a non-200 statuscode (show message in that case if possible, otherwise "unknown error...")
• Adjust tests