fix(error): improve the current error handler in DSP-APP (DSP-1911)

[kilchenmann]

resolves DSP-1911

You can test it as follows:

• Start the app with the test-server configuration
• Open the following page: http://0.0.0.0:4200/search/fulltext/Stygimoloch
• If you're logged-in, you should see the correct error message from JS-Lib instead the 503 error mentioned in DSP-1910

Or

• Start the app with your local api stack environment
• Turn off the API
• You should see the 503 server error

[mdelez]

Works great!