test/py: exit(1) if there are problems running py.test

The test/py/test.py wrapper script catches exceptions thrown when
exec()ing py.test in order to print a helpful error message. However,
the exception handling code squashes the exception and so the script
exits with a non-zero exit code, leading callers to believe that it
passed. Fix this.
Signed-off-by: NStephen Warren <swarren@nvidia.com>

test/py/test.py

@@ -30,3 +30,4 @@ except:
     print >>sys.stderr, '''
 exec(py.test) failed; perhaps you are missing some dependencies?
 See test/py/README.md for the list.'''
+    sys.exit(1)