BUG: Reading a corrupt sigmet file raises IOError

Reading a Sigmet file which is corrupt before the end of the first sweep
raises a IOError.  Only a warning is issue when the file contains one or more
complete sweeps.

Calling the read_sigmet function with debug=True will provide details of where
the file is corrupt.
Merge pull request #192 from jjhelmus/bug_sigmet_corrupt

BUG: Reading a corrupt sigmet file raises IOError