Better error message for bad parameter default

[Saif807380]

Added a try block to check whether the default value provided to an option with multiple=True is a valid iterable. If the value is not a valid iterable, a TypeError with a better error message is raised.

• fixes Traceback when CLI option has bad default could be more helpful #1749

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.

[effigies]

Ah, I wish I'd followed this. Would it be possible to identify which argument generated the TypeError? It looks like there's a self.name that could be used.

[davidism]

Both the errors raised in type_cast_value should be in __init__ instead. The composite error should be removed, and both places that value is iterated in type_cast_value should raise a BadParameter error instead of TypeError.

BadParameter adds the parameter name before the message.