Show more convenient error if uploaded raster is not readable by gdal

[jirik]

Example of currently produced async error:

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/celery/app/trace.py", line 412, in trace_task
    R = retval = fun(*args, **kwargs)
  File "/code/src/layman/make_celery.py", line 36, in __call__
    return self.run(*args, **kwargs)
  File "/code/src/layman/layer/filesystem/tasks.py", line 93, in refresh_gdal
    gdal.assert_valid_raster(input_path)
  File "/code/src/layman/layer/filesystem/gdal.py", line 64, in assert_valid_raster
    color_interp = get_color_interpretations(input_path)
  File "/code/src/layman/layer/filesystem/gdal.py", line 46, in get_color_interpretations
    for band_id in range(1, dataset.RasterCount + 1):
AttributeError: 'NoneType' object has no attribute 'RasterCount'