DO NOT MERGE 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: Ia90531e3455d4888cfac6510cab879c50883be53
(cherry picked from commit 575d68a48e)

tools/releasetools/common.py

@@ -62,6 +62,9 @@ class Options(object):
     self.source_info_dict = None
     self.target_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()