Richer error messages for 412 responses to signing requests

[siladu]

When signing with slashing protected enabled, maySign can return false resulting in a 412 API response:
https://github.com/ConsenSys/web3signer/blob/17d253bd617d5f54a7d7fe1a4b7849aa9922a125/core/src/main/java/tech/pegasys/web3signer/core/service/http/handlers/signing/eth2/Eth2SignForIdentifierHandler.java#L153-L160

The error messaging assumes it is due to slashing protection rules, however there are several reasons: GVR missing, database error, validator being disabled.

Original context: #520 (comment)

...However, from a code reading POV is the name SLASHING_PROTECTION_ENFORCED and log message "Signing not allowed due to slashing protection rules failing" slightly misleading for this case?
...
Yeah it is a bit misleading. The handler assumes it's due to the slashing protection rules failing but that's not always true. Could be GVR missing, database error, validator being disabled. Perhaps we should be returning a richer error answer from the slashing protection with errors that return an error message or code? Think this should be another ticket though.