[xml mode] Leave old style intact when adding error style

Fixes a problem with matchtags getting confused.

Closes #1841

mode/xml/xml.js

@@ -76,7 +76,7 @@ CodeMirror.defineMode("xml", function(config, parserConfig) {
         tagName = "";
         var c;
         while ((c = stream.eat(/[^\s\u00a0=<>\"\'\/?]/))) tagName += c;
-        if (!tagName) return "error";
+        if (!tagName) return "tag error";
         type = isClose ? "closeTag" : "openTag";
         state.tokenize = inTag;
         return "tag";
@@ -109,7 +109,8 @@ CodeMirror.defineMode("xml", function(config, parserConfig) {
       type = "equals";
       return null;
     } else if (ch == "<") {
-      return "error";
+      stream.match(/^\/?\w*\/?>/);
+      return "tag error";
     } else if (/[\'\"]/.test(ch)) {
       state.tokenize = inAttribute(ch);
       state.stringStartCol = stream.column();
@@ -298,7 +299,9 @@ CodeMirror.defineMode("xml", function(config, parserConfig) {
         }
       }
       state.startOfLine = false;
-      return setStyle || style;
+      if (setStyle)
+        style = setStyle == "error" ? style + " error" : setStyle;
+      return style;
     },
 
     indent: function(state, textAfter, fullLine) {