Allow to abort Upload with MultipartEncoderMonitor

[paulmueller]

For uploading large files with MultipartEncoderMonitor, it would be helpful to be able to abort the upload. I think this could be done conveniently with the callback function, i.e. instead of https://github.com/requests/toolbelt/blob/70c1abaab7c14072e8214c3eb8e9c585f4d98a95/requests_toolbelt/multipart/encoder.py#L400-L404 one could do something like:

 def read(self, size=-1): 
     string = self.encoder.read(size) 
     self.bytes_read += len(string) 
     cb = self.callback(self)
     if cb is None:
         # The callback function normally returns `None`.
         return string 
     else:
         # If it returns something else, the upload is stopped by raising an Exception.
         # This exception can be caught in a try-except statement wrapping around
         # the `requests.post(...` call.
         raise MultipartEncoderAbort("MultipartEncoder streaming aborted by callback: {}".format(cb))

What do you think? Could something like this be included in requests_toolbelt or should I subclass MultipartEncoderMonitor in my own projects for this purpose?

[sigmavirus24]

The hard thing to reason about here is how this interacts with Requests and the remote server. As suggested, this doesn't seem wise.