Add root exception for all pint errors

[c-w]

Many libraries follow the pattern where there's a single root exception that can be caught to identify any exception the library throws which makes it very easy to isolate issues related to the library. This pull request adds such a root exception.

Usage example:

import pint
ureg = pint.UnitRegistry()

try:
  # do something involving ureg
except pint.PintError:
  # here we know for sure that the error came from pint

• Closes # (insert issue number)
  • This is a developer ergonomics improvement that came to mind while working with the library so I didn't open an issue for it.
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
  • I didn't see any existing references to exception handling in the docs so I didn't add anything related to the new root exception.
• Added an entry to the CHANGES file

[hgrecco]

I think this a good idea. What do you think @jules-ch ?

[jules-ch]

That's a good idea ! 👍