Checkstyle produces invalid xml if rustfmt failed

[ahal]

STR
Find a file with a line that's too long:
$ rustfmt --write-mode=checkstyle file.rs

...
<error line="1624" severity="warning" message="Should be `                   .clone())`" /><error line="1633" severity="warning" message="Should be `                   .clone())`" /><error line="1664" severity="warning" message="Should be `            Nullable::Value(ref x) =&gt; Json::Object(try!(x.to_marionette())),`" /><error line="1684" severity="warning" message="Should be `        Ok(try_opt!(self.to_json().as_object(),`" /><error line="1684" severity="warning" message="Should be `                    ErrorStatus::UnknownError,`" /><error line="1684Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/prefs.rs:114: line exceeded maximum length (maximum: 100, found: 112) (sorry)
Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/prefs.rs:115: line exceeded maximum length (maximum: 100, found: 109) (sorry)
Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/marionette.rs:540: line exceeded maximum length (maximum: 100, found: 122) (sorry)
Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/marionette.rs:658: line exceeded maximum length (maximum: 100, found: 119) (sorry)
Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/marionette.rs:1442: line exceeded maximum length (maximum: 100, found: 108) (sorry)
Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/capabilities.rs:224: line exceeded maximum length (maximum: 100, found: 122) (sorry)
Rustfmt failed at /home/ahal/hg/mozilla-central/testing/geckodriver/src/capabilities.rs:229: line exceeded maximum length (maximum: 100, found: 105) (sorry)
" severity="warning" message="Should be `                    &quot;Expected an object&quot;)`" /><error line="1684" severity="warning" message="Should be `                   .clone())`" /><error line="1696" severity="warning" message="Should be `            FrameId::Element(ref x) =&gt; {`" /><error line="1696" severity="warning" message="Should be `                data.insert(&quot;element&quot;.to_string(), Json::Object(try!(x.to_marionette())))`" /><error line="1696" severity="warning" message="Should be `            }`" /><error line="1696" severity="warning" message="Should be `            FrameId::Null =&gt; None,`" /></file><file name="/home/ahal/hg/mozilla-central/testing/geckodriver/src/prefs.rs"></file></checkstyle>

The "Rustfmt failed" messages seem to get printed right in the middle of the last "error" tag, causing the xml parser to fail. Ideally stuff like "line too long" would be incorporated into the xml tree, but at the very least would be nice if they got dumped at the end.