Unable to get hyperlink in add.error code, In trigger.

I have seen codes in forum, but wrkg out, as its giving like output text, i need in clickable format.

i tried this:

o.addError('ERROR TRM100: There is already an identical record: <a href=\'https://cs2.salesforce.com/' + Acctid.id + '\'>Record Number ' + Acctid.id + '</a>');