ENH: Added file path existence check for read_hdf

closes  #7715 

Added simple check for file path existence. It might be more advisable to deprecate this function and separate file buffer and file path read functionality, but the current form matches the rest of the API.

I wasn't sure if there was any tests I needed to add, but if there is, let me know.
Maybe say something to the effect: previously if you tried to read from a non-existant file an empty file would be created.