Fixed implementation of path alias settings.

Now report an error if path is already aliased to another location.

examples/demo/site/themes/bootstrap/assets/css/style.css

@@ -76,3 +76,9 @@ ul.horizontal li {
   border-width: 2px 1px 0;
   padding: 2px 7px 1px;
 }
+
+#message li.error {
+  background-color: #f99;
+  border: solid 1px red;
+  padding: 5px 2px 5px 2px;
+}

examples/demo/site/themes/bootstrap/assets/scss/style.scss

@@ -81,3 +81,8 @@ ul.horizontal {
 		}
 	}
 }
+#message li.error {
+	background-color: #f99;
+	border: solid 1px red;
+	padding: 5px 2px 5px 2px;
+}