Api: Only log unexpected exceptions in ApiException

[Daverball]

Commit message

Api: Only log unexpected exceptions in ApiException

TYPE: Bugfix

Checklist

• I have performed a self-review of my code

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.49%. Comparing base (7780915) to head (f3681f4).
Report is 1 commits behind head on master.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/onegov/api/models.py	89.88% <100.00%> (+0.06%)	⬆️

... and 1 file with indirect coverage changes

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