feat: Improve error messages for invalid submissions

[Koc]

Right now it is really hard to understand for end users why form can't be submitted. This PR aims to improve validation messages.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 43.38%. Comparing base (f8fca97) to head (a82cc79).
Report is 13 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #2533      +/-   ##
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- Coverage     43.40%   43.38%   -0.03%     
+ Complexity      882      881       -1     
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  Files            77       77              
  Lines          3359     3356       -3     
============================================
- Hits           1458     1456       -2     
+ Misses         1901     1900       -1     

[Chartman123]

Nice one :)

[susnux]

I dont think this makes sense, what is the use case?
It is nothing shown to end users, that should already be done in the frontend and if it is not then there is a bug in the frontend that should be fixed.

So this is for developers that use the API?
In this case we should refactor the function at all (validateSubmission) to be void and throw instead on error.
Either InvalidArgumentException if we keep this developers only I would prefer this.
Otherwise an \OCP\HintException which would allow translated error messages for frontend - but here again we should never allow to submit something invalid so a frontend validation is wrong.