Streamline API error output

API already has a default way of failing (e.g. when passing invalid arguments).
We shouldn't be outputting flow-specific errors differently: this only makes it
harder to process errors, since we then have to deal with 2 different formats
we have to interpret.

Change-Id: I77a8cf1ae1bb73f211e982da209ae554393a4471
Merge "Streamline API error output"