Keyper crashes on transient RPC gateway error during subscription setup with an unwrapped error

[ylembachar]

Summary

When the gateway's backend is unavailable, the Keyper exits at startup with an unwrapped error and crash-loops.

Error: json: cannot unmarshal object into Go value of type string

Root cause

The RPC gateway (gateway.fm) returns a 503 inside the result field on eth_subscribe instead of using the
JSON-RPC error field (violates JSON-RPC 2.0 §5.1):

{"jsonrpc":"2.0","result":{"code":503,"message":"server unavailable"},"id":1}

go-ethereum (rpc/handler.go:409-419) sees no top-level error, so it decodes result into the subscription-id string and fails. The error returns unwrapped from SubscribeNewHead through unsafehead.go:30-33 to the service runner to Cobra, killing the process. The same applies to WatchKeyperSetAdded (keyperset.go:69-71).

Fix

We already resubscribe on restart, so the issue is only that the subscription-setup error is returned bare and unloggable. Wrap it at the setup path in the syncers:

• unsafehead.go (~line 30): wrap the SubscribeNewHead error
• keyperset.go (~line 69): wrap the WatchKeyperSetAdded error

return errors.Wrap(err, "failed to subscribe to new heads")

Apply the same to the other syncers' Start methods (ShutterStateSyncer, EonPubKeySyncer).