hgweb: add error template to json so it won't crash

mercurial/templates/json/map

@@ -241,3 +241,6 b" indexentry = '\\{"
   "lastchange": {lastchange|json},
   "labels": {labels|json}
   }'
+error = '\{
+  "error": {error|utf8|json}
+  }'

tests/test-hgweb-json.t

@@ -2183,6 +2183,16 b' help/{topic} shows an individual help to'
     "topic": "phases"
   }
 
+Error page shouldn't crash
+
+  $ request json-changeset/deadbeef
+  404 Not Found
+  
+  {
+    "error": "unknown revision 'deadbeef'"
+  }
+  [1]
+
 Commit message with Japanese Kanji 'Noh', which ends with '\x5c'
 
   $ echo foo >> da/foo