Improve exception message when attempting to connect to an invalid DSN

[dnicolodi]

Connecting to a source with a misspelled or missing scheme, results in a ModuleNotFoundError:

  File ".../lib/python/site-packages/beanquery/__init__.py", line 49, in attach
    source = importlib.import_module(f'beanquery.sources.{scheme}')
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/opt/local/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/importlib/__init__.py", line 90, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "<frozen importlib._bootstrap>", line 1387, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1324, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'beanquery.sources.'

Catch this exception and turn it into something more user friendly.