This is apparently unofficially deprecated, and we can do things a bit
more straightforwardly by using ParserOutput::addJsConfigVars() to
communicate the error messages to the JS.
This also takes the opportunity to move "ext.scribunto", which is mostly
about errors, to "ext.scribunto.errors".
Bug: T75618
Change-Id: I1577dab2dab1bd79cb127879de141fdbb8963aeb