accomodate grpc v1.40.0 402 response code / error formatting changes

[dstadulis]

Dysfunction

As mentioned in lightninglabs/pool#451 and other PRs google.golang.org/grpc v1.40.0 has changed their 402 response code by improving the parsing from the HTTP/2 stack (see grpc/grpc-go#4474). Consequently when the response code is output by gRPC, it's format has changed.

Necessary Remediation

This output change necessitates an update to L402 to continue to be compatible. Short term mitigation is to downgrade to grpc v1.39.0

Scope

Scope of engineering work to fix should be easy