User file upload: gracefully handle non-images

In some cases user upload may look like an image, but it isn't. The code handling thumbnail generation will fail and abort the upload. This can be handled gracefully allowing upload without thumb generation:

OverflowError: Python int too large to convert to C ssize_t
  File "django/core/handlers/exception.py", line 56, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/csrf.py", line 55, in wrapped_view
    return view_func(*args, **kwargs)
  File "django/views/generic/base.py", line 103, in view
    return self.dispatch(request, *args, **kwargs)
  File "rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
  File "rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "contextlib.py", line 81, in inner
    return func(*args, **kwds)
  File "baserow/api/decorators.py", line 105, in func_wrapper
    return func(*args, **kwargs)
  File "baserow/api/user_files/views.py", line 61, in post
    user_file = UserFileHandler().upload_user_file(request.user, file.name, file)
  File "baserow/core/user_files/handler.py", line 233, in upload_user_file
    image = Image.open(stream)
  File "PIL/Image.py", line 3277, in open
    im = _open_core(fp, filename, prefix, formats)
  File "PIL/Image.py", line 3263, in _open_core
    im = factory(fp, filename)
  File "PIL/ImageFile.py", line 117, in __init__
    self._open()
  File "PIL/PsdImagePlugin.py", line 127, in _open
    self.layers = _layerinfo(_layer_data, size)
  File "PIL/PsdImagePlugin.py", line 241, in _layerinfo
    t = _maketile(fp, m, bbox, 1)
  File "PIL/PsdImagePlugin.py", line 287, in _maketile
    file.seek(offset)

KeyError: 'PSD'
  File "django/core/handlers/exception.py", line 56, in inner
    response = get_response(request)
  File "django/core/handlers/base.py", line 197, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "django/views/decorators/csrf.py", line 55, in wrapped_view
    return view_func(*args, **kwargs)
  File "django/views/generic/base.py", line 103, in view
    return self.dispatch(request, *args, **kwargs)
  File "rest_framework/views.py", line 509, in dispatch
    response = self.handle_exception(exc)
  File "rest_framework/views.py", line 469, in handle_exception
    self.raise_uncaught_exception(exc)
  File "rest_framework/views.py", line 480, in raise_uncaught_exception
    raise exc
  File "rest_framework/views.py", line 506, in dispatch
    response = handler(request, *args, **kwargs)
  File "contextlib.py", line 81, in inner
    return func(*args, **kwds)
  File "baserow/api/decorators.py", line 105, in func_wrapper
    return func(*args, **kwargs)
  File "baserow/api/user_files/views.py", line 61, in post
    user_file = UserFileHandler().upload_user_file(request.user, file.name, file)
  File "baserow/core/user_files/handler.py", line 258, in upload_user_file
    self.generate_and_save_image_thumbnails(image, user_file, storage=storage)
  File "baserow/core/user_files/handler.py", line 168, in generate_and_save_image_thumbnails
    thumbnail.save(thumbnail_stream, image.format)
  File "PIL/Image.py", line 2419, in save
    save_handler = SAVE[format.upper()]

Also, reported by user: https://community.baserow.io/t/cant-upload-psd-file-to-baserow-is-it-a-bug-or-a-design-limitation/5477