Error-handling best practice

[LiviaValenti]

Is it best practice requiring importing from the package twice? Should the package even export error functionality instead of just throwing errors with messages to the developer to handle?

Originally posted by @Xtrah in #29 (comment)

[eivindkopperud]

I've had a look at this now, and I feel like it makes more sense to define CustomError (or something else) inside NTNUI/opptak, and not export it from NTNUI/ntnui-tools. There are some places where several responses with status code 400 can be sent, e.g.:

1. I try to log in with wrong credentials, and get a 400 error
2. I try to log in with correct credentials, but I have no committee memberships, and I get a 400 error

It would be nice to have a errorCode: INVALID_CREDENTIALS and a errorCode: MISSING_COMMITTEE_MEMBERSHIP, such that the frontend can catch both and display a better message. Currently I get a "the user is not found" for both the cases.

This logic does not really have anything to do with ntnui-tools, so CustomError should be defined in NTNUI/opptak.