The multipart document handling in the proxy is consumed via iteration,
but the error handling code is not consistent with how it applies
conversions of IO errors/timeouts and retry failures to StopIteration.
In an effort to make the code more obvious and easier to debug and
maintain I've added comments and additional tests as well as tightening
up StopIteration exception handling.
Co-Authored-By: Alistair Coles <alistairncoles@gmail.com>
Change-Id: I0654815543be3df059eb2875d9b3669dbd97f5b4
4a4d899680