Steps to reproduce (on mw.org, or any other wiki with the most recent version):

1. Click the icon for the link dialog
2. Enter some random nonsense into the the field for the page title
3. Enter an invalid title, such as <

Expected result:
In step 2 the page should be flagged as missing, in step 3 as invalid.
Actual result:
In step 2 the page is flagged as existing, in step 3 a JavaScript error is thrown (this only happens in my test wiki, on mw, no error is thrown, but the page is flagged as existing, which isn't as bad as an error, but wrong nevertheless).

Probably both a regression caused by linting all the js files. (https://gerrit.wikimedia.org/r/#/c/14014/)

Version: unspecified
Severity: normal