Fixes unserialize() related warnings

After the last [commit](https://github.com/scribu/wp-scb-framework/commit/19e8854a2569e14c5b69793e4562519547761283), the error handler sometimes displays unserialize() related warnings:
"Warning: unserialize() expects parameter 1 to be string, array given"

Replacing 'unserialize()' with 'maybe_unserialize()' fixes it.