Change the cache partition size check into warnings.

For some old builds, we may not define cache partition size. Change the
exception into a warning to make the script backward compatible.

Change-Id: Ie94c7fbb1a9f3a7db3f16e8d845e493a534aac5b

tools/releasetools/common.py

@@ -57,6 +57,9 @@ class Options(object):
     self.extras = {}
     self.info_dict = None
     self.worker_threads = None
+    # Stash size cannot exceed cache_size * threshold.
+    self.cache_size = None
+    self.stash_threshold = 0.8
 
 
 OPTIONS = Options()