Bail with an error on a non-existent jobroot_dir

If the user gives a config location that does not exist, raise an error.
We don't do makedirs calls later - and depending on the location it
wouldn't necessarily be appropriate to.

Change-Id: If4ab428682d2130a0b0f1cc63e57f9aa51b7e241
diff --git a/zuul/cmd/executor.py b/zuul/cmd/executor.py
index 931639f..bd14b74 100755
--- a/zuul/cmd/executor.py
+++ b/zuul/cmd/executor.py
@@ -121,6 +121,10 @@
         if self.config.has_option('zuul', 'jobroot_dir'):
             self.jobroot_dir = os.path.expanduser(
                 self.config.get('zuul', 'jobroot_dir'))
+            if not os.path.isdir(self.jobroot_dir):
+                print("Invalid jobroot_dir: {jobroot_dir}".format(
+                    jobroot_dir=self.jobroot_dir))
+                sys.exit(1)
         else:
             self.jobroot_dir = tempfile.gettempdir()