Better error handling

[ue71603]

You can study those cases and get better error messages.

Basically: There is no PT leg, so I guess it should not try to generate a nova request:

from test_configuration.py

No result, but no result is ok

READFILE.append("input/input_walk_only.xml")
READFILE.append("input/input_sharing_only.xml")
READFILE.append("input/input_odv_alone.xml")
READFILE.append("input/input_in_the_past_not_handeled_well_in_Preisauskunft.xml")

[tuxalp]

Now we have the centralized error handling (package api.errors) that introduces an error handler, and a couple of exceptions mapped to api error responses by this handler.

But there are certainly still a lot places in the code where error responses can be improved with the help of this new handler, probably also with new more specific exceptions. That's why I would keep this issue open.