Loosen angle tolerance in DiffractionLibrary.get_library_entry() from 1e-5 to 1e-2

[hakonanes]

Description of the change

Fix #173.

Also changed three diffraction library tests raising errors to also check for the specific error message.

Note that the one line that is not covered in this PR is covered in #182.

Progress of the PR

• [n/a] Docstrings for all functions
• Unit tests with pytest for all lines
• Clean code style by running black

For reviewers

• The PR title is short, concise, and will make sense 1 year later.
• New functions are imported in corresponding __init__.py.
• New features, API changes, and deprecations are mentioned in the
  unreleased section in CHANGELOG.rst.

[pc494]

LGTM, thanks @hakonanes very lazy of me not to pick this one up myself.