Avoid error when intro or outro logging fields are configured.

doc/changelog.txt

@@ -6,6 +6,8 @@ Fixes:
 - checking: Do not parse URL CGI part recursively, avoiding maximum
   recursion limit errors.
   Closes: SF bug #3096115
+- logging: Avoid error when logger fields "intro" or "outro" are
+  configured.
 
 Changes:
 - cmdline: Don't log a warning if URL has been redirected.

linkcheck/logger/__init__.py

@@ -222,7 +222,7 @@ class Logger (object):
         """
         Return translated part name.
         """
-        return _(Fields[name])
+        return _(Fields.get(name, u""))
 
     def spaces (self, name):
         """