handle non-existent cloud probability masks

[ungarj]

Here is the traceback of two processes failing because the cloud probability mask is not here:

MapcheteTaskFailed('tile_task_z13-(13-631-6)_result raised a Exception("FileNotFoundError(\'s3://sentinel-s2-l2a/tiles/1/K/BS/2025/4/4/0/QI_DATA/MSK_CLDPRB_60m.jp2 not found and cannot be opened with rasterio: ObjectNotFound: The specified key does not exist.\')")')
  File "/app/.venv/lib/python3.12/site-packages/mapchete/commands/execute.py", line 183, in execute
    for count, task_info in enumerate(

  File "/app/.venv/lib/python3.12/site-packages/mapchete/processing/base.py", line 288, in execute
    for task_info in dask_graph(

  File "/app/.venv/lib/python3.12/site-packages/mapchete/processing/execute.py", line 107, in dask_graph
    for future in executor.compute_task_graph(

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/dask.py", line 223, in compute_task_graph
    for future in task_manager.finished_futures():

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/dask.py", line 374, in finished_futures
    yield self.executor.to_mfuture(
          ^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/base.py", line 139, in to_mfuture
    mfuture.raise_if_failed()

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/future.py", line 234, in raise_if_failed
    raise MapcheteTaskFailed(

MapcheteTaskFailed('tile_task_z13-(13-573-2047)_result raised a Exception("FileNotFoundError(\'s3://sentinel-s2-l2a/tiles/1/L/AH/2025/7/29/0/QI_DATA/MSK_CLDPRB_60m.jp2 not found and cannot be opened with rasterio: ObjectNotFound: The specified key does not exist.\')")')
  File "/app/.venv/lib/python3.12/site-packages/mapchete/commands/execute.py", line 183, in execute
    for count, task_info in enumerate(

  File "/app/.venv/lib/python3.12/site-packages/mapchete/processing/base.py", line 288, in execute
    for task_info in dask_graph(

  File "/app/.venv/lib/python3.12/site-packages/mapchete/processing/execute.py", line 107, in dask_graph
    for future in executor.compute_task_graph(

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/dask.py", line 223, in compute_task_graph
    for future in task_manager.finished_futures():

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/dask.py", line 374, in finished_futures
    yield self.executor.to_mfuture(
          ^^^^^^^^^^^^^^^^^^^^^^^^^

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/base.py", line 139, in to_mfuture
    mfuture.raise_if_failed()

  File "/app/.venv/lib/python3.12/site-packages/mapchete/executor/future.py", line 234, in raise_if_failed
    raise MapcheteTaskFailed(