Improve error messages when no granules available

[franTarkenton]

The script retrieves granules for modis imagery, sometimes the data is not available and no data gets downloaded. When that happens and the next process that attempts to mosaic the granules, an error is generated.

The code should detect this condition and provide a more informative error message like

"no granules found for the current date range... "
or
"the granule directory for the date range (directory here) doesn't contain any information"