Better error messages for corrupt Creer merge tags

[JacobFischer]

So normally tags look like this:

        // <<-- Creer-Merge: render -->>
        // render where the Port is
        // <<-- /Creer-Merge: render -->>

However, if a character accidentally gets deleted, e.g.:

        // <<-- Creer-Merge: render -->
        // render where the Port is
        // <<-- /Creer-Merge: render -->>

The > character at the end of the first line is missing

Creer blows up trying to merge them, because the tags are malformed. It should tell the user something like an end tag was found without an opening tag.

[JacobFischer]

Bumping this as it happened again this semester with mis-matched tags and the errors were once again hard to discern this was the bug