improve fetch error handling

[derhuerst]

Currently, almost all places in the code base assume that

1. network requests are successful – They often fail because state is missing and also don't display an error message.
2. fetch() rejects on non-2xx responses, which is not the case.

Proposed Changes

This PR introduces a helper function fetchWithErrors that rejects with a FetchError on non-2xx responses, and changes some of the network utility functions in the code base to use it.

Pull Request Check List

• A reasonable set of unit tests is included
• Console does not show new warnings/errors – It does show different (the actual) errors now.
• Changes are documented or they are self explanatory

Review

• Read and verify the code changes
• Test the functionality by running the UI locally with all popular browsers available in your platform
• Check that the implementation matches the design, when such one is defined in a Jira issue
• Merge the pull request