Improve error message for invalid density array

[santisoler]

Add size of density array and number of tesseroids on the error message
when they mismatch in order to make clearer what's failing.

The improvement of such error messages was triggered after #98.

Reminders

• Run make format and make check to make sure the code follows the style guide.
• Add tests for new features or tests that would have caught the bug that you're fixing.
• Add new public functions/methods/classes to doc/api/index.rst.
• Write detailed docstrings for all functions/methods.
• If adding new functionality, add an example to the docstring, gallery, and/or tutorials.

[leouieda]

Nice 🍻 Thanks, Santi