tiny nit: default case for an exhaustive switch should generate a severe log entry (and possibly panic on non-production builds)

Description of the bug

tiny nit re making it harder to introduce errors later.

default case for a switch that is intended to be exhaustive should log and/or panic. i.e., emulate C/C++ -Wswitch -Wswitch-enum behavior

https://github.com/oasislabs/ekiden/blob/221f43394d4469bd71ab72302feec15f67c0944e/go/tendermint/api/api.go#L59

Steps to reproduce

Expected behavior

If a future code change introduces a new enum (what the const CodeXXX is emulating) but forgets to update the String() method for Code values, we would detect it quickly in tests (where hopefully a new high-severity log entry would be noticed) or in a dev environment (panics). Not sure if golang builds permit prod vs debug builds, so panics might be too scary.

Environment

• OS:
• Versions of relevant software and tools:

Additional information
Anything else you think would be useful; including context and suggested fixes.

Start at go/tendermint/api/api.go around line 59 and inspect the switch used by Code.String(). Determine how exhaustive enum handling should report an unexpected value, including whether production and non-production builds need different behavior. Done means future unmapped Code values produce the agreed severe log entry or panic behavior.