Add jobstart() errors to debug log

This should make issues like #14 significantly easier to debug.

Author: vimpostor

autoload/lumen.vim

@@ -112,6 +112,7 @@ func lumen#fork_job()
 		let options = #{on_stdout: function('lumen#on_stdout'), on_stderr: function('lumen#on_stderr'), on_exit: function('lumen#on_exit')}
 		silent! let s:job = jobstart(command, options)
 		if s:job == 0 || s:job == -1
+			call lumen#debug#log_err('jobstart() failed: ' . v:errmsg)
 			call lumen#on_exit(0, 256 - s:job, 0)
 		endif
 	else