fix(time): raise ValueError instead of KeyError for unsupported minimum_unit

[MeiSiristhebest]

Summary

When humanize.precisedelta, naturaldelta, or naturaltime are called with an unsupported unit string for minimum_unit (or suppress in precisedelta), humanize previously raised an opaque KeyError.

This PR changes the behavior to raise a clear ValueError: Minimum unit '<unit>' not supported, consistent with existing error messages when non-minimum unit enum members (e.g. years for naturaldelta) are supplied.

Behavior Change

# Before
>>> precisedelta(timedelta(seconds=1), minimum_unit="weeks")
KeyError: 'WEEKS'

# After
>>> precisedelta(timedelta(seconds=1), minimum_unit="weeks")
ValueError: Minimum unit 'weeks' not supported

Changes

• Added _minimum_unit_or_raise helper in src/humanize/time.py.
• Updated naturaldelta and precisedelta to catch invalid unit names and raise ValueError from None.
• Added unit tests in tests/test_time.py for unknown minimum_unit and suppress values across precisedelta, naturaldelta, and naturaltime.