Add error handling for WMS client

[joker234]

This error happened probably because of issues in the tile server and should be handled correctly in SMT.

sketch-map-tool-celery-1    | [2023-02-10 10:29:30,206: ERROR/ForkPoolWorker-5] Task sketch_map_tool.tasks.generate_sketch_map[49cbe1bc-9598-4bef-ae16-b76f8f3ec485] raised unexpected: UnidentifiedImageError('cannot identify image file <_io.BytesIO object at 0x7f7487b948b0>')
sketch-map-tool-celery-1    | Traceback (most recent call last):
sketch-map-tool-celery-1    |   File "/home/smt/.cache/pypoetry/virtualenvs/sketch-map-tool-7oog9W5A-py3.10/lib/python3.10/site-packages/celery/app/trace.py", line 451, in trace_task
sketch-map-tool-celery-1    |     R = retval = fun(*args, **kwargs)
sketch-map-tool-celery-1    |   File "/opt/smt/sketch_map_tool/__init__.py", line 46, in __call__
sketch-map-tool-celery-1    |     return self.run(*args, **kwargs)
sketch-map-tool-celery-1    |   File "/opt/smt/sketch_map_tool/tasks.py", line 51, in generate_sketch_map
sketch-map-tool-celery-1    |     map_image = wms_client.as_image(raw)
sketch-map-tool-celery-1    |   File "/opt/smt/sketch_map_tool/wms/client.py", line 48, in as_image
sketch-map-tool-celery-1    |     return Image.open(response.raw)
sketch-map-tool-celery-1    |   File "/home/smt/.cache/pypoetry/virtualenvs/sketch-map-tool-7oog9W5A-py3.10/lib/python3.10/site-packages/PIL/Image.py", line 3186, in open
sketch-map-tool-celery-1    |     raise UnidentifiedImageError(
sketch-map-tool-celery-1    | PIL.UnidentifiedImageError: cannot identify image file <_io.BytesIO object at 0x7f7487b948b0>