when downloading new MODIS files, check that each HDF file is valid

[robinkraft]

Inspired by #299.

A checker function in the updater script should open each HDF file after it's downloaded and thereby catch any obvious issues before they showed up in a Cascalog query. Only after it was successfully opened should the upload to S3 commence. On error, the problem file should be noted and its filename included in the notification email.

fname = "/mnt/files/MOD13A1.A2012305.h09v09.005.2012322025347.hdf"
f = gdal.Open(fname)
ERROR 4: `/mnt/files/MOD13A1.A2012305.h09v09.005.2012322025347.hdf' not recognised as a supported file format.

if type(f) == NoneType:
    raise Exception("Invalid file: %s" % fname)
else:
    startUpload(fname)