web change hook: rename exception variable name

Use a more descriptive name of the cought instance of ValueError
exception. The generic 'err' was redefined as a local function
on line 81.

This fixes pylint error 'E0102 (function-redefined)' in this file.

master/buildbot/status/web/change_hook.py

@@ -60,9 +60,9 @@ def render_POST(self, request):
 
         try:
             changes, src = self.getChanges( request )
-        except ValueError, err:
-            request.setResponseCode(400, err.args[0])
-            return err.args[0]
+        except ValueError, val_err:
+            request.setResponseCode(400, val_err.args[0])
+            return val_err.args[0]
         except Exception, e:
             log.err(e, "processing changes from web hook")
             msg = "Error processing changes."