Some pages cannot be saved due to invalid page terms encoding

When attempting to save an asset page it would raise a mysql error when trying to update the page terms.

the asset in question had a .txt thumbnail - but that file seemed to contain a binary.

Looks like we have some assets that have broken .txt thumbnails.

We should either check the text for a valid encoding before attempting to insert it into the page terms or we should at least catch the error and make sure processing can proceed.