Fixed "TypeError: invalid data" when executables run by exec exited with non-zero code.

The original error was because process.stderr only allows writing Strings or Buffers, but status is a Number.

Also, the message parameter of the "error" handler is just the amalgam of all output written to stderr. This is already logged by the "stderr" handler, so there's no need to write it to process.stderr again.
Merge pull request #1500 from Arnavion/exec-process-stderr-number

Fixed "TypeError: invalid data" when executables run by exec exited with non-zero code.