Add error message formatting section to UX guidance

[ishan372or]

This PR adds a section to the UX guidance documentation covering error message formatting principles and examples.

 1.Based on the 2020 UX error message proposal

 2.Includes structured examples for dependency conflict errors

 3.Aims to improve clarity and usability of pip error messages

[notatallshaw]

Hi and thanks for the time you spent on making this PR.

However, I believe the UX guidance in the pip documentation has a specific history and lineage that was contracted and involved user studies. Other maintainers may want to comment but I don't think we will be looking to modify or add to it without a very strong justification.

Further, unrelated changes should not be part of a PR, e.g. the update to .gitignore and exceptions.py.

[ishan372or]

Thank you for the feedback. That makes sense — I wasn't aware the UX guidance was based on contracted research. I’ll close this PR and look for more appropriate issues to contribute to. Appreciate your time!