Unwrap python exceptions in crud procedures

[joshelser]

I realized that we are giving an ugly python stacktrace back to the user after the pyCRUD migration work.

In the top of the procedure, we should catch any exception and return back the exception's text rather than raise an unhandled exception. Now, the user sees:

Traceback (most recent call last):
  File "_udf_code.py", line 12, in run_delete
  File "/home/udf/27672521534251782/crud.zip/crud/wh_sched.py", line 285, in delete_warehouse_schedule
    raise ValueError("Cannot delete the last schedule for a warehouse.")
ValueError: Cannot delete the last schedule for a warehouse.
 in function DELETE_WAREHOUSE_SCHEDULE with handler run_delete

They could simply see the following instead:

Cannot delete the last schedule for a warehouse.

Related: we indicate an "error" right now if the procedure does not return NULL. Maybe we should be returning a proper exception? Not sure what is "normal" when it comes to non-SQL procedures.