On cron write fail ask the user to check the output is valid cron syntax

The old error message asks users to try running `whenever` without
arguments to ensure the schedule file is valid. If this fails then the
user can look in their schedule file for an error. But what if running a
plain `whenever` succeeds? This updated message acknowledges
that a successful run of `whenever` does not mean the schedule file 
is valid and asks them to check the _output_ is syntactically valid and
does not contain errors.