Report error from fd to user

[dvzubarev]

I stumbled upon this error in fd.
But before this fix no actual error from fd was reported. Actually it was not helpful error in dirvish-fd-proc-sentinel:

Debugger entered--Lisp error: (wrong-type-argument number-or-marker-p nil)
  dirvish-fd--parse-output()
  dirvish-fd-proc-sentinel(#<process fd> "exited abnormally with code 1\n")

Im not sure that it is the best way to handle errors, but at least It can save time when debugging original error.

[alexluigit]

Thanks for this!