fix(mint): fail with UNAVAILABLE when the connection drops

[arrangabriel]

Fail with UNAVAILABLE when the Mint connection drops

A connection that was established and then went away failed with gRPC error status UNKNOWN.
gRPC reserves UNAVAILABLE for a transient transport failure.
A request that was already in flight when the connection dropped returned a {:error, "the connection is closed"} string rather than a %GRPC.RPCError{}.

Changes

• Connection-closed errors are now built as %GRPC.RPCError{status: unavailable} in the connection process, so every path out of a dropped connection carries the struct.
• handle_errors_receive_data/2's error wrapping now goes through (rpc_error/1).
  • An error that already carries a status passes through untouched.
  • Mint transport errors and connection-gone HTTP/2 reasons map to UNAVAILABLE. Reasons that cannot be classified still fail with UNKNOWN.