Add name, value info to convert_from_llsd<> specialization LL_ERRS.

Prior to this, you could get a viewer crash whose ERROR: message said only:
convert_from_llsd<std::string>: Invalid string value
This gave no hint as to *which value* was wrong, or where to go fix it.
Ironically, each convert_from_llsd<> specialization already has the
control_name and LLSD value in hand; added these to each such LL_ERRS message.