(bug 45887) Improve error popup discoverability

The Scribunto "Script error" can be clicked, which is indicated by a CSS
cursor:pointer style. To make it more discoverable, also have it show an
underline on hover (as links do by default).

And while we're messing with it, let's remove that "style" attribute and
use the already-existing CSS class to assign the styles.

Bug: 45887
Change-Id: Ibbbf0ed268efd78e4b7844cc2639fe154ee3ab8a
Merge "(bug 45887) Improve error popup discoverability"