Missing files should raise an exception

[kdeloach]

So as not to be confused by exceptions that occur as a direct consequence of missing files. There's no point to continue executing the RWD algorithm if the source data cannot be loaded.

Ex.

Error running Point_Watershed_Function
Traceback (most recent call last):
  File "/opt/rwd/src/api/main.py", line 66, in run_rwd
    output_path,
  File "/opt/rwd/rwd/Rapid_Watershed_Delineation.py", line 211, in Point_Watershed_Function
    Output_dir)
  File "/opt/rwd/rwd/RWSDelin_Utilities.py", line 516, in Get_Watershed_Attributes
    Basin_length=extract_value_from_raster(plen_file_with_path,Outlet_Point)
  File "/opt/rwd/rwd/RWSDelin_Utilities.py", line 221, in extract_value_from_raster
    gt = src_ds.GetGeoTransform()
AttributeError: 'NoneType' object has no attribute 'GetGeoTransform'