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
diff --git a/tools/releasetools/blockimgdiff.py b/tools/releasetools/blockimgdiff.py
index a007f81..a32c6b1 100644
--- a/tools/releasetools/blockimgdiff.py
+++ b/tools/releasetools/blockimgdiff.py
@@ -461,7 +461,7 @@
       if free_string:
         out.append("".join(free_string))
 
-      if self.version >= 2:
+      if self.version >= 2 and common.OPTIONS.cache_size is not None:
         # Sanity check: abort if we're going to need more stash space than
         # the allowed size (cache_size * threshold). There are two purposes
         # of having a threshold here. a) Part of the cache may have been
@@ -502,10 +502,16 @@
 
     if self.version >= 2:
       max_stashed_size = max_stashed_blocks * self.tgt.blocksize
-      max_allowed = common.OPTIONS.cache_size * common.OPTIONS.stash_threshold
-      print("max stashed blocks: %d  (%d bytes), limit: %d bytes (%.2f%%)\n" % (
-          max_stashed_blocks, max_stashed_size, max_allowed,
-          max_stashed_size * 100.0 / max_allowed))
+      OPTIONS = common.OPTIONS
+      if OPTIONS.cache_size is not None:
+        max_allowed = OPTIONS.cache_size * OPTIONS.stash_threshold
+        print("max stashed blocks: %d  (%d bytes), "
+              "limit: %d bytes (%.2f%%)\n" % (
+              max_stashed_blocks, max_stashed_size, max_allowed,
+              max_stashed_size * 100.0 / max_allowed))
+      else:
+        print("max stashed blocks: %d  (%d bytes), limit: <unknown>\n" % (
+              max_stashed_blocks, max_stashed_size))
 
   def ComputePatches(self, prefix):
     print("Reticulating splines...")