RPC: document error codes for getblock

[teor2345]

Is your feature request related to a problem? Please describe.

The getblock documentation does not document the error code used when a block is not available.

But lightwalletd checks for the exact error code -8:
https://github.com/adityapk00/lightwalletd/blob/c1bab818a683e4de69cd952317000f9bb2932274/common/common.go#L251-L254

Describe the solution you'd like

Document the error code returned when there is no block with that height or hash.

Alternatives you've considered

Maybe lightwalletd could document this requirement, or provide a better error message?