Add another check for a failed googleapiclient upgrade.

This adds one more check for a failed 1.2 -> 1.3 upgrade, specifically in the
`apiclient` import (which was the only import available in 1.2). Even combined
with the check in setup.py, this won't catch everything, but it now covers all
the most common cases.
diff --git a/apiclient/__init__.py b/apiclient/__init__.py
index 9577a5e..5db22a1 100644
--- a/apiclient/__init__.py
+++ b/apiclient/__init__.py
@@ -2,6 +2,17 @@
 
 import googleapiclient
 
+try:
+  import oauth2client
+except ImportError:
+  raise RuntimeError(
+      'Previous version of google-api-python-client detected; due to a '
+      'packaging issue, we cannot perform an in-place upgrade. To repair, '
+      'remove and reinstall this package, along with oauth2client and '
+      'uritemplate. One can do this with pip via\n'
+      '  pip install -I google-api-python-client'
+  )
+
 from googleapiclient import channel
 from googleapiclient import discovery
 from googleapiclient import errors