Need to deal with Moose exceptions being objects

[tla]

The exception raising mechanism in Text::Tradition::Error tends to assume that whatever error it is asked to raise is a string. Moose has turned all of its exceptions into objects, which causes an exception if we try to treat it as a string when throwing an exception (confused yet?) We need to add a check for this.

[tla]

But actually this was fixed in commit 11ccb53 already.