Display more precise error messages (e.g. when trying to log in with not existing mail adress)

[veitmo]

User story

1. As a {HR employee}
2. I want {precise error messages which tell me what went wrong}
3. So that {I know how to solve errors and correctly use DIdentity}

Acceptance criteria

• check for all error messages which are potentially displayed to the user:
  • all error messages clearly state the problem that occured (e.g. email-address does not exist)
  • all error messages are written in an understandable way, so that people without a technical background are able to understand it
  • bug 401 Error is not properly visualized #274 is included in this issue

Definition of done (DoD)

• code peer-reviewed (by at least one team member)
• acceptance criteria met
• integrated into a clean build
• documentation updated
• code is well-formatted (according to our norms) and structure
• Code supports all functions/cases and all responses defined in the API
• no severe bugs during main functionalities (program should not stop executing due to these bugs, e.g. runtime exception in Docker)
• Bill of Materials updated if necessary
• 70% of the tests pass
• important functions shall be tested
• Code has been deployed via Docker to the server

DoD general criteria

• Feature has been fully implemented
• Feature has been merged into the mainline
• All acceptance criteria were met
• Product owner approved features
• All tests are passing
• Developers agreed to release