feat: display maintenance page in case of a 503 http status error

[SGrueber]

PR Type

[ ] Bugfix [x] Feature [ ] Code style update (formatting, local variables) [ ] Refactoring (no functional changes, no API changes) [ ] Build-related changes [ ] CI-related changes [ ] Documentation content changes [ ] Application / infrastructure changes [ ] Other:

What Is the Current Behavior?

If the ICM server is in maintenance mode REST requests respond with an 503 http status code, but the client doesn`t handle these errors properly.

Issue Number: Closes #

What Is the New Behavior?

The pwa shows a maintenance page if a REST request respond with a 503 error.

Does this PR Introduce a Breaking Change?

[ ] Yes [x] No

Other Information

Needs the following ICM fix concerning an incomplete response header: AB#76259

AB#75906