python-style error messages in tensor/

[fchapoton]

make sure error messages in tensor start with lower-case

also a little bit of pep8 cleanup in one file

:memo: Checklist

• [x] The title is concise and informative.
• [x] The description explains in detail what this PR is about.

[fchapoton]

apparemment, non. J'ai un peu la flemme. Je peux faire celui dans format_utilities.py

[dcoudert]

Ce serait un bon début ;)

[fchapoton]

voilà