Update pipeline module import to no longer fail on ImportError, but i…

…f the module doesn't exist

python/src/mapreduce/base_handler.py

@@ -23,16 +23,19 @@
 
 import httplib
 import logging
+import pkgutil
+import importlib
 
 try:
   import json
 except ImportError:
   import simplejson as json
 
-try:
-  from mapreduce import pipeline_base
-except ImportError:
-  pipeline_base = None
+pipeline_base = None
+
+if pkgutil.find_loader('mapreduce.pipeline_base') is not None:
+  pipeline_base = importlib.import_module('mapreduce.pipeline_base')
+
 try:
   # Check if the full cloudstorage package exists. The stub part is in runtime.
   import cloudstorage