Improve error messages when no granules available

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"

Start by tracing the Python script from MODIS granule retrieval to the process that mosaics the granules. Reproduce the empty-data condition for a date range, then make the failure identify that no granules were found or that the granule directory is empty.

Written by the indexing model from the issue text.