Fix uncaught / uninformative errors on data build with no raw files

[nsfabina]

Example:

  File "/home/nfabina/rsCNN/rsCNN/data_management/data_core.py", line 93, in build_or_load_rawfile_data
    self.config.raw_files.boundary_files)
  File "/home/nfabina/rsCNN/rsCNN/configuration/sections.py", line 517, in check_input_file_validity
    num_r_bands_per_file = [gdal.Open(x, gdal.GA_ReadOnly).RasterCount for x in r_file_list[0]]
  File "/home/nfabina/rsCNN/rsCNN/configuration/sections.py", line 517, in <listcomp>   
    num_r_bands_per_file = [gdal.Open(x, gdal.GA_ReadOnly).RasterCount for x in r_file_list[0]]
AttributeError: 'NoneType' object has no attribute 'RasterCount'