chore: enable ruff EM

[henryiii]

This is the ruff EM check, based on flake8-errmsg (my package). It changes tracebacks from this:

Traceback (most recent call last):
  File "tmp.py", line 2, in <module>
    raise RuntimeError(f"{sub!r} is incorrect")
RuntimeError: 'Some value' is incorrect

to this:

Traceback (most recent call last):
  File "tmp.py", line 3, in <module>
    raise RuntimeError(msg)
RuntimeError: 'Some value' is incorrect

This avoids printing the error message twice, sometimes with placeholders. It also tends to format a little nicer.

It's a ruff autofix, so was easy to apply. We don't have to if people don't like it.

[brettcannon]

I personally don't care either way.