remove double code
always try to run just the command, even if the abscommand() is False

if Frescobaldi cant find the executable, it should not assume that
the operating system wont be able to run it.
always return a job
sorry, but as Frescobaldi cant decide on its own whether "lilypond" can of can not be executed we can only error out after an unsuccesful start... :-)

that said, we will add a helpful message after an unsuccesful run.
add job.failedToStart(), job.error and job.success attributes