Returning underlying cause for User Defined Functions

[wsargent]

The sqlite3 client takes user defined functions and replaces the text with "user-defined function raised exception`" so it's not apparent what's gone wrong:

Unexpected error: user-defined function raised exception

As mentioned in https://code.djangoproject.com/ticket/29500 and https://stackoverflow.com/questions/45824209/how-to-get-an-error-kind-from-sqlite-create-function/45834923#45834923 the workaround for this is to enable callback tracebacks:

sqlite3.enable_callback_tracebacks(True)

It would be nice if https://sqlite-utils.datasette.io/en/stable/python-api.html#registering-custom-sql-functions either included a reference to enable_callback_tracebacks or if registering a user defined function set this flag automatically.

[simonw]

I'm nervous to set this by default in the Python library because it's a global setting, so I can't isolate it to just code that is being called by a sqlite_utils method.

The new sqlite-utils convert command from #251 DOES turn it on - I think having it on by default for a CLI tool that uses user-defined functions makes sense:

sqlite-utils/sqlite_utils/cli.py

Line 2006 in e83aef9

sqlite3.enable_callback_tracebacks(True)

I like the suggestion to include this in the documentation. I'll do that!