configitems: register the 'web.errorlog' config

mercurial/configitems.py

@@ -622,6 +622,9 b" coreconfigitem('web', 'description',"
 coreconfigitem('web', 'encoding',
     default=lambda: encoding.encoding,
 )
+coreconfigitem('web', 'errorlog',
+    default='-',
+)
 coreconfigitem('worker', 'backgroundclose',
     default=dynamicdefault,
 )

mercurial/hgweb/server.py

@@ -283,7 +283,7 b' class MercurialHTTPServer(_mixin, httpse'
         self.prefix = prefix
 
         alog = openlog(ui.config('web', 'accesslog'), ui.fout)
-        elog = openlog(ui.config('web', 'errorlog', '-'), ui.ferr)
+        elog = openlog(ui.config('web', 'errorlog'), ui.ferr)
         self.accesslog = alog
         self.errorlog = elog