Print clear error if a user is tryng to load a .mat file version < 7.3

Probably we can check if the .mat file is supported by looking at the starting bytes of the file, see:

~~~
def is_hdf5(file_path):
with open(file_path, 'rb') as f:
signature = f.read(8)
return signature == b'\x89HDF\r\n\x1a\n'
~~~

Based on https://docs.hdfgroup.org/hdf5/v1_14/_f_m_t3.html#Superblock, it seems that this indeed a way to quickly check if a given file is hdf5 .

Start at the code path that loads .mat files and inspect how unsupported file versions currently fail. Use the HDF5 signature check described in the issue to distinguish supported files, then verify that older versions produce a clear error instead of the existing failure.

Written by the indexing model from the issue text.