FindHDF5: Tolerate failure during compiler interrogation

Check that the file that is accessed via file(STRINGS...) truly exists.

This is not the case in some EMSCRIPTEN/WebASM builds where we want to overwrite the HDF5 find logic manually. The missing check here prevents that one can overwrite the search logic in exotic environments.

X-ref: https://github.com/pyodide/pyodide/pull/2169
Backport: release
Topic-rename: FindHDF5-info-binary-exists