Avoid blanket catching of `Exception`

[brassy-endomorph]

For example:

https://github.com/scidsg/hushline/blob/75ee6e0e58e71f014aedf4381dbd377d38691de7/app.py#L1192-L1205

This should catch an IntegrityError and check that the violated unique key matches what we expect. The current broad exception catch will make it more difficult to test and will hide programming errors and other bugs