agent: return gRPC Internal error on submission failure

[leoluk]

Stack from ghstack:

• bridge: log submission for cross-submissions to Solana #54 bridge: log submission for cross-submissions to Solana
• bridge: do not resubmit submitted VAAs during aggregation #53 bridge: do not resubmit submitted VAAs during aggregation
• bridge: do not log errors for duplicate VAA submissions #52 bridge: do not log errors for duplicate VAA submissions
• bridge: have all nodes submit VAAs to Solana #51 bridge: have all nodes submit VAAs to Solana
• bridge/pkg/solana: retry VAA submission on transient errors #48 bridge/pkg/solana: retry VAA submission on transient errors
• agent: return gRPC Internal error on submission failure #47 agent: return gRPC Internal error on submission failure
• bridge: move initial guardian set fetching to pkg/ethereum/watcher.go #46 bridge: move initial guardian set fetching to pkg/ethereum/watcher.go
• bridge: propagate panics from runnables #45 bridge: propagate panics from runnables
• bridge: in-place debugging using dlv #44 bridge: in-place debugging using dlv

This allows us to distinguish between temporary and permanent failure.

Unless we check the instruction error that occured, we can't know
whether the submission error is a permanent failure and Internal
is therefore the appropriate code to use.