fix: enable tracebacks for "user"/custom sqlite functions

[ThinkChaos]

Description

A bit niche but I tried setting my bareasc prefix to an empty string,
and was getting an obtuse error. This should help make clearer what is
happening when queries fail.

The exception is not properly raised up the stack in the first place
because it happens across 2 FFI boundaries: the DB query
(Python -> SQLite), and the custom DB function (SQLite -> Python).
Thus Python cannot forwarded it back to itself through SQLite, and it's
treated as an "unraisable" exception.

We could override sys.unraisablehook to not print anything for the
original exception, and store it in a global for the outer Python
interpreter to fetch and raise properly, but that's pretty hacky,
limited to a single DB instance and query at once, and risks swallowing
other "unraisable" exceptions.
Instead we just tell the user to look above for what Python prints.

Sample output:

Exception ignored in: <function unidecode_expect_ascii at 0x7f7fa20bb060>
Traceback (most recent call last):
  File "site-packages/unidecode/__init__.py", line 60, in unidecode_expect_ascii
    bytestring = string.encode('ASCII')
                 ^^^^^^^^^^^^^
AttributeError: 'bytes' object has no attribute 'encode'
Traceback (most recent call last):
  File "site-packages/beets/dbcore/db.py", line 988, in query
    cursor = self.db._connection().execute(statement, subvals)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
sqlite3.OperationalError: user-defined function raised exception

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "site-packages/beets/__main__.py", line 9, in <module>
    sys.exit(main())
             ^^^^^^
  File "site-packages/beets/ui/__init__.py", line 1865, in main
    _raw_main(args)
  File "site-packages/beets/ui/__init__.py", line 1852, in _raw_main
    subcommand.func(lib, suboptions, subargs)
  File "site-packages/beets/ui/commands.py", line 1599, in list_func
    list_items(lib, decargs(args), opts.album)
  File "site-packages/beets/ui/commands.py", line 1594, in list_items
    for item in lib.items(query):
                ^^^^^^^^^^^^^^^^
  File "site-packages/beets/library.py", line 1695, in items
    return self._fetch(Item, query, sort or self.get_default_item_sort())
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "site-packages/beets/library.py", line 1673, in _fetch
    return super()._fetch(model_cls, query, sort)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "site-packages/beets/dbcore/db.py", line 1301, in _fetch
    rows = tx.query(sql, subvals)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "site-packages/beets/dbcore/db.py", line 991, in query
    raise DBCustomFunctionError()
beets.dbcore.db.DBCustomFunctionError: beets defined custom SQLite function failed; see the other errors above for details

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.