Use backend errors in toast notifications

[somebody1234]

Pull Request Description

Adds the message returned by the backend to the toast notification, so that the user gets a more detailed error message.

Important Notes

• Not sure what's the best way to properly test this, as for a lot of the backend endpoints there is no easy way to (even intentionally) trigger them from the frontend, because the frontend does a decent amount of validation as well.

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.