Raise ImportError instead of exiting, if buildbot.test can't be impor…

…ted.

This apparently causes issue with 'pydoc -k', and is in general not nice.
Trial still fails with a sensible error message.

This is a slave-side reflection of commit 126e023

slave/buildslave/test/__init__.py

@@ -14,7 +14,6 @@
 # Copyright Buildbot Team Members
 
 import sys
-import os
 
 import twisted
 from twisted.trial import unittest
@@ -58,6 +57,5 @@ def nopatch(self, *args):
     import mock
     mock = mock
 except ImportError:
-    print >>sys.stderr, ("\nBuildbot tests require the 'mock' module; "
-                         "try 'pip install mock'")
-    os._exit(1)
+    raise ImportError("Buildbot tests require the 'mock' module; "
+            "try 'pip install mock'")