Fix unhandled exception when getting the HDU data fails (typically in the case of the file being truncated; see https://github.com/astropy/astropy/issues/541)

git-svn-id: http://svn6.assembla.com/svn/pyfits/trunk@2265 ed100bfc-0583-0410-97f2-c26b58777a21
Add a changelog entry for this fix
Merge pull request #1332 from embray/fits-comp-unhandled-exception

Fix unhandled exception in astropy.io.fits.compression module