Add an error message for the case fsautocomplete.exe is not built

[kmyk]

I got the same error to #106 and it was caused by forgetting building fsautocomplete.exe. Adding build = "make fsautocomplete" to my dein.toml solved this problem. The current error message BrokenPipeError: [Errno 32] Broken pipe is unhelpful, so this pull request replace it.