Documentation for snapshot transactions mentions:

If streaming a chunk fails due to a “resumable” error, Session.read() retries the StreamingRead API reqeust, passing the resume_token from the last partial result streamed.

This is the correct thing to do but the client actually does not seem to be doing this. I looked around a little bit in the code and I could find anyplace where it is retrying the read with the resume token.
This is the place where the stream would throw an error if it broke:
https://github.com/GoogleCloudPlatform/google-cloud-python/blob/9d9b6c0708ae6553458ad6107e5ad8efb23762e8/spanner/google/cloud/spanner/streamed.py#L132

I dont see any exception handling happening here and it looks like that the error would just be propagated back to the user.