Refuse to run MAF if non UTF-8 characters are received from MythTV

[billmeek]

This is a result of #219. If the backend is run using a character set other
than UTF-8, then the JSON parser can fail on characters outside of the
'normal' ASCII set, e.g. é.

MAF should detect the invalid character set (e.toString().contains( "Invalid UTF-8")), alert the user and refuse to run until the backend is properly configured.