Audit use of panicky-macros

[zbuc]

In #4444 @cratelyn noted that the use of the unimplemented!() macro in RPCs can allow a malicious client to crash the node software.

I performed a brief search through the codebase and saw several other calls to unimplemented elsewhere. We should audit the codebase for panicky macros (panic, unimplemented, todo, etc.) and ensure they're unable to crash node software.