Spec: Clean up error responses in REST protocol

[rdblue]

This cleans up some of the error response documentation in the REST protocol.

• Documents 500 and 504 specifically in the table commit route
• Adds stack to error responses for sending server-side stack traces (optional)
• Adds 419 to all routes to signal that authentication has timed out
• Adds 503 to all routes for a back-pressure signal

[danielcweeks]

LGTM

[kbendick]

A few small nits, but overall this looks good to me.

Thank you @rdblue!

[rdblue]

Thanks for reviewing, @kbendick and @danielcweeks!