Fixed implementation of path alias settings.

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

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;
+}