Fixes #369. Clarify fixture assertions

[mathomp4]

Closes #369

This just cleans up a bit for when a components.yaml doesn't have a fixture.

Before:

...
  File "/Users/mathomp4/.homebrew/brew/Cellar/mepo/2.4.0/libexec/lib/python3.12/site-packages/mepo/registry.py", line 49, in __validate
    assert num_fixtures == 1
           ^^^^^^^^^^^^^^^^^
AssertionError

Now:

...
  File "/Users/mathomp4/mepo/src/mepo/registry.py", line 51, in __validate
    raise ValueError("At least one fixture must be defined in the registry")
ValueError: At least one fixture must be defined in the registry

I'll let @pchakraborty see if this should be a ValueError or something different.