Specify in API docs that API errors return a JSON error object
Specify in API docs that API errors return a JSON error object

This only changes the documentation. #1243 should be closed before the
next release so that the implementation matches the docs.

mharkus/Sia,ChrisSchinnerl/Sia,rivine/rivine,mtlynch/Sia,ChrisSchinnerl/Sia,NebulousLabs/Sia,mnsl/Sia,ericflo/Sia,rivine/rivine,ericflo/Sia,MrStrong/Sia,mtlynch/Sia,mharkus/Sia,MrStrong/Sia,NebulousLabs/Sia,mnsl/Sia