Return error for msgpack-rpc requests

- We don't handle msgpack-RPC requests yet, but we need to return
  an error response
New signal NeovimConnector::processExited

- When launching Neovim through ::spawn we need to know when the process
  exits, this signal is emitted when that happens
For Gcc/Clang be pedantic

- Brought back -Wall and -Wextra
Remove unnecessary connection to discoverMetadata