Bug 1027950 - [Settings][Find My Device] Do not attach handlers inside onerror callback

See Bugzilla bug for discussion.
Interestingly, it turns out that both XUL[1] and the html5 spec[2][3] consider the `disabled` attribute to be a boolean attribute, so its absence, not its value, is what should determine if the el is disabled. Going to leave it as-is, let me know if I'm misunderstanding the spec, and I'll fix it next time around :-)

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Attribute/disabled
[2] http://www.whatwg.org/specs/web-apps/current-work/#enabling-and-disabling-form-controls:-the-disabled-attribute
[3] http://www.w3.org/TR/html5/forms.html#enabling-and-disabling-form-controls:-the-disabled-attribute