Handle KeyError better in the service error handler functions

[lorenzo-cavazzi]

This is a follow-up to #2303 (especially to this conversation)

There are a number of different reasons for a KeyError to happen. We should handle this case better in the decorators in renku/service/views/error_handlers.py (or improve where the errors are originally raised, where possible).

In general, trying to catch all the errors we can identify (especially user-generated) and raise a more descriptive error is a good strategy. Everything un-catched could raise a ProgramSomethingError explaining in the devMessage how the parameters were wrong. If something ends up as a Program error but it shouldn't, we can always handle the case as soon as we identify it.

[Panaetius]

https://github.com/SwissDataScienceCenter/renku-python/pull/2303/files/4ded6eec1a006a8323ca172f3221d03b691bf091#diff-2b20acc34bf9f18ae845d1ca22b4145c140f2ba5e9a04c42467998d1a154fe2dR243