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/ota_from_target_files.py

@@ -1634,7 +1634,7 @@ def main(argv):
 
     cache_size = OPTIONS.info_dict.get("cache_size", None)
     if cache_size is None:
-      raise RuntimeError("can't determine the cache partition size")
+      print "--- can't determine the cache partition size ---"
     OPTIONS.cache_size = cache_size
 
     if OPTIONS.incremental_source is None: