[yordis]

• The Mint adapter reports a dropped connection or a cancelled request as a bare string, while the Gun adapter surfaces transport failures as %GRPC.RPCError{}, so callers pattern matching on RPCError crash depending on which adapter is configured.
• The gRPC status code spec defines a client that cannot reach the server as UNAVAILABLE, which lets callers apply their usual retry policy instead of special-casing adapter strings.
• Follows the direction of fix(mint): return raw connect errors, not strings #584, which made the connect path stop returning strings.