Improve exception traceback readability

[hugovk]

New users often struggle with tracebacks, especially when they're long.

This is a small change to help simplify the output and avoid repetition in tracebacks.

Before

Here's a simple example from https://github.com/henryiii/flake8-errmsg:

sub = "Some value"
raise RuntimeError(f"{sub!r} is incorrect")

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

After

sub = "Some value"
msg = f"{sub!r} is incorrect"
raise RuntimeError(msg)

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