Improve error handling in nightly when CHPL_NIGHTLY_LOGDIR is not set.

* Report error and exit with non-zero code if CHPL_NIGHTLY_LOGDIR is not set
  (or empty) in -cron mode.
* Try to create the logdir, if it does not exist already (for both -cron and
  -debug modes).
* Check that directory is accessible in both -cron and -debug modes.

Some of these things were already done in -debug mode.
Merge branch 'master' of github.com:chapel-lang/chapel into improve-error-checking-in-nightly
Merge branch 'master' of github.com:chapel-lang/chapel into improve-error-checking-in-nightly
Merge pull request #659 from thomasvandoren/improve-error-checking-in-nightly

Improve error checking in nightly

Add error checks to nightly for the CHPL_NIGHTLY_LOGDIR value that would
have made the issue fixed by #658 easier to realize.