API cleanup

[michbarsinai]

While working towards JavaOne'16, where we present our API implementation, I found various inconsistencies. This issue is used for tracking their fixes.

[michbarsinai]

d813105: Should be "Illegal command translates to 403 FORBIDDEN.

[djbrooke]

Heard from @michbarsinai that this one is ready for code review!

#3381 is the PR.