[#1927] Raise exception on invalid input in indent

[alex-harvey-z3q]

Before this change, the indent filter would raise a confusing error like the following if an unexpected input type was sent e.g. a dict:

unsupported operand type(s) for +=: 'dict' and 'str'"

This updates the code to fail gracefully and adds a new test.

• fixes #1927

Checklist:

• [x] Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• [ ] Add or update relevant docs, in the docs folder and in code.
• [ ] Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• [ ] Add .. versionchanged:: entries in any relevant code docs.
• [x] Run pre-commit hooks and fix any issues.
• [x] Run pytest and tox, no tests failed.