Better handle XML parsing issues

### Repro

```
const xml = new DOMParser().parseFromString('', 'text/xml');
try {
const geojson = togeojson(xml);
console.log(geojson);
} catch (err) {
console.error(err)
}
```

### Actual result

```
{ type: 'FeatureCollection', features: [] }
```

### Expected result

Exception thrown with error speaking to the fact that the XML is bad.

### Ideas for solution

```
var d = new DOMParser().parseFromString('bar', 'text/xml');
Array.from(d.querySelectorAll('parsererror > div')).forEach(err => console.log(err.innerText));
```

Outputs: `error on line 1 at column 19: Opening and ending tag mismatch: foo line 0 and foop`

Start at the togeojson(xml) entry point and inspect how the DOMParser result exposes parsererror elements. Handle malformed XML so the conversion reports an exception containing the parser error, then verify it with the malformed XML repro and the expected thrown-error behavior.

Written by the indexing model from the issue text.