Improve error message when opening files

[mantid-builder]

Original Reporter: @FedeMPouzols

When you try to Load() a file that is not readable you'll get an error like this:

/home/fedemp/test/build-mantid/bin/mantid/simpleapi.pyc in Load(Filename, **kwargs)
     87     algm = _create_algorithm_object('Load')
     88     _set_logging_option(algm, kwargs)
---> 89     algm.setProperty('Filename', filename) # Must be set first
     90     # Remove from keywords so it is not set twice
     91     try:

ValueError: HierarchicalFileDescriptor::isHierarchical - Unable to open file '/home/fedemp/mantid/TrainingCourseData/LOQ48097.raw'

which can be a bit confusing. See how to make it more clear, suggest that you might not have read permission on the file, etc.

[mantid-builder]

This issue was originally trac ticket 11113