Replace bash shell completion version error with warning

[superatomic]

Replaces instances of raise RuntimeError(...) with echo(..., err=True) in _check_version().

• fixes Don't fail with error when bash is not found or too old #2574

Checklist:

• 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.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[superatomic]

#2581 should be merged first before this PR is merged.

[superatomic]

It might be worth removing the warning when Bash is not on the system at all, so the behavior is more in line with Zsh and Fish.

[davidism]

I'm not planning to change it more. This is geared towards people using it directly, not towards other build systems.

[superatomic]

Makes sense. Thank you for your review and your work! I appreciate it.