XML: No error reported for missing namespace definition

## Sample

1. Open https://ace.c9.io/build/kitchen-sink.html?doc=XML (Make sure that "Enable Behaviors" is activated)
2. Replace the content with
```xml

```

## What is the expected result?
An error indication is shown that there is no namespace definition for the prefix `abc`. The developer should add `xmlns:abc="myNameSpaceDefinition"` in the same node or at the parent level.

## What happens instead?
There is **no error** indication on the left side.

When trying to parse the XML document (e.g. by a framework), it throws a vague parse error. This is fine as long as the document is short, but in real projects, XML files can become quite large and finding the cause of the error becomes difficult.

This could be avoided when the error is shown early in the design time.