Replace "alias dothis" with "alias dothis {"
Perhaps $errorline would be more useful then $errorsource, both would be cool though.

I really hope this will be added to mIRC. I could use it to trace a clear path an error took.
For example,
alias a {
return $b
:error
echo -s error: $error ( $+ $script $+ , Line $errorline $+ )
; In this example it would echo
; error: $b : $c : Some Error (ScriptName, Line 2)
}
alias b {
return $c
:error
; $errorline here would equal 9
/throwerror $ $+ b : $error
}
alias c {
/throwerror $ $+ c : Some Error
}