I think the (at line 10, column 18) part is equally helpful.

I'd even go as far as to suggest the most likely cause, and how to fix it, maybe similar to:

poetry-core cannot parse toml file at '/home/luckydonald/code/poetry/pyproject.toml'.
  TOMLDecodeError: Cannot overwrite a value (at line 10, column 18)
This is often caused by a duplicate entry.

Couldn't find a good way to check for that duplication from the exception alone.
I'd just do

if e.msg = 'Cannot overwrite a value':
  txt += "\nThis is often caused by a duplicate entry."

This give everyone a clear cause to look out for.
Yes, that might "break" if the text changes, but if it does, it's will be just as cryptic as before, nothing lost.

This give everyone a clear cause to look out for.
Yes, that might "break" if the text changes, but if it does, it's will be just as cryptic as before, nothing lost.

That's ok with me.

Couldn't find a good way to check for that duplication from the exception alone. I'd just do

if e.msg = 'Cannot overwrite a value':
  txt += "\nThis is often caused by a duplicate entry."

Done. Getting the message out of a ValueError (which TOMLDecodeError is a thin subclass of) is a bit less clean though. Did it by converting to a string.

This give everyone a clear cause to look out for. Yes, that might "break" if the text changes, but if it does, it's will be just as cryptic as before, nothing lost.

The unit test would also fail, so it would be caught and updated eventually.

Thanks for this @mscheifer!