Fix crash when root directory does not exist

We would crash trying to get the prefix for the files if our root
directory did not exist (I found this out by accidentally passing the
wrong path for a nightly build). This fixes that problem by deferring if
we don't find any files.

We should probably also just put the source path into the cloner, too,
since that's the only thing that uses it.

srcloner.py

@@ -275,6 +275,16 @@ class StoneRidgeCloner(object):
 
         sys.exit(0)
 
+    def exit_and_maybe_defer(self, deferred_message):
+        next_attempt = self.attempt + 1
+        if next_attempt > self.max_attempts:
+            logging.error('Unable to get build results for %s. '
+                    'Cancelling run.' % (self.srid,))
+        else:
+            self.defer()
+            logging.debug(deferred_message)
+        sys.exit(1)
+
     def run(self):
         files = self._gather_filelist(self.path)
         if not self.nightly:
@@ -295,15 +305,8 @@ class StoneRidgeCloner(object):
             # is ready for us to download.
             for d in subdirs:
                 if d not in files:
-                    next_attempt = self.attempt + 1
-                    if next_attempt > self.max_attempts:
-                        logging.error('Unable to get build results for %s. '
-                                'Cancelling run.' % (self.srid,))
-                    else:
-                        self.defer()
-                        logging.debug('Run %s not available: retry later' %
-                                (d,))
-                    sys.exit(1)
+                    self.exit_and_maybe_defer(
+                            'Run %s not available: retry later' % (d,))
 
             dist_path = '/'.join([self.path, subdirs[0]])
             dist_files = self._gather_filelist(dist_path)
@@ -316,6 +319,10 @@ class StoneRidgeCloner(object):
 
             files = dist_files
 
+        if not files:
+            self.exit_and_maybe_defer(
+                    'No files found for %s: retry later' % (self.srid,))
+
         self.prefix = self._get_prefix(files)
 
         # Make sure our output directory exists