Sane API errors

Description

Refactors the API to use exceptions instead of explicit error response generation.

Additions

• An abstract API exception class that contains its corresponding API error code
• 14 new exceptions implementing this class
• An exception handler that translates these exceptions into error responses
• Error codes now have a HTTP response code that should be returned if that error happens
• findExisting() functions in the repositories that throw an exception if the stored value could not be found

Changes

• API controllers use findExisting where possible
• Validation throws correct exceptions
• API fields are limited in length
• Some endpoints return different status codes in error conditions as these were incorrect previously

Test and Review

Resolves #176 (closed)
Resolves #94 (closed)
Improves #156

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files present in the MR

• The continuous integration has no problems with the MR

• The MR is filled in as requested (including labels, milestones, and reviewers)

• The documentation is up-to-date

• All nullable parameters are marked as such