BUG: fix error message when trying to load a Parthenon dataset without h5py installed #8443

Job	Run time
Image tests	6m 41s
unit tests: py3.13 on macos-latest (pytest)	13m 13s
unit tests: py3.10.3 on ubuntu-20.04 (pytest)	10m 41s
answer tests: py3.10 on ubuntu-latest (nose)	13m 55s
unit tests: py3.10 on ubuntu-latest (nose)	13m 59s
unit tests: py3.13 on ubuntu-latest (pytest)	16m 40s
unit tests: py3.x on ubuntu-latest (pytest)	9m 51s
unit tests: py3.13 on windows-latest (pytest)	18m 13s
	1h 43m 13s

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