better error for unmatched closing tag (#321)

src/parse/state/tag.js

@@ -90,6 +90,8 @@ export default function tag ( parser ) {
 
 		// close any elements that don't have their own closing tags, e.g. <div><p></div>
 		while ( parent.name !== name ) {
+			if ( parent.type !== 'Element' ) parser.error( `</${name}> attempted to close an element that was not open`, start );
+
 			parent.end = start;
 			parser.stack.pop();
 

test/parser/error-unmatched-closing-tag/error.json

@@ -0,0 +1,8 @@
+{
+	"message": "</div> attempted to close an element that was not open",
+	"loc": {
+		"line": 1,
+		"column": 0
+	},
+	"pos": 0
+}

test/parser/error-unmatched-closing-tag/input.html

@@ -0,0 +1 @@
+</div>