Server-side hard errors gives unexpected error messages on client side

[GoogleCodeExporter]

What steps will reproduce the problem?
1. Connect to the sercer using the boar:// protocol.
2. Start a commit that is large enough to cause an out of disk space error on 
the server. (although any critical error on the server is likely to show the 
same problem)

What is the expected output? What do you see instead?
There should be a nice message saying something has gone terribly wrong on the 
server. Instead, there is an incomprehensible assertion that "Client tried to 
make a RPC call during data upload."

Original issue reported on code.google.com by ekb...@gmail.com on 3 Jan 2015 at 9:33