esgdrs: change "Invalid or corrupted NetCDF file" message if read permission is lacking

[alaniwi]

If an input data file does not have read permission for the user running esgdrs, the message obtained is "Invalid or corrupted NetCDF file". It would be good to change this to something more informative.

[alaniwi]

These are the places in the code.

esgf-prepare/esgprep/utils/misc.py

Lines 50 to 55 in 6705fc8

	def __enter__(self):
	try:
	self.nc = Dataset(self.path, self.mode)
	except IOError:
	raise InvalidNetCDFFile(self.path)
	return self.nc

esgf-prepare/esgprep/utils/custom_exceptions.py

Lines 18 to 27 in e3a749b

	class InvalidNetCDFFile(Exception):
	"""
	Raised when invalid or corrupted NetCDF file.
	"""
	def __init__(self, path):
	self.msg = "Invalid or corrupted NetCDF file."
	self.msg += "\n<file: '{}'>".format(path)
	super(self.__class__, self).__init__(self.msg)

It probably needs an additional custom exception type, and an explicit permissions test using os.access.

Correction: it might be possible to inspect the IOError exception for the error code, and if it is the one that corresponds to a permissions problem, then simply re-raise the IOError instead of the custom exception. Something like...

import errno

def __enter__(self): 
     try: 
         self.nc = Dataset(self.path, self.mode) 
     except IOError as exc: 
         if exc.errno == errno.EACCES:
             raise
         else:
             raise InvalidNetCDFFile(self.path) 
     return self.nc