Fix alt error msg from 2i timeout

The code could receive two different error messages depending on where
the 2i timeout occurred (webmachine process vs in the index FSM). Some
extra error handling code in the streaming requests was added since it
was useful when figuring out this bug.
Make 2i http multipart parsing more robust

Previously it died in all sorts of ways if the packages were merged or
split into weird bits by the network or network library. Now the results
are concatenated and the whole thing parsed in the end.
Merge pull request #407 from basho/eas-2i-timeout-and-multipart

Improve 2i timeout and http multipart parsing in 2i tests