Separated construction of exception message in json/cast.h.
Separated construction of exception message in json/cast.h.

takei-yuya/pficommon,pfi/pficommon,retrieva/pficommon,retrieva/pficommon,takei-yuya/pficommon,retrieva/pficommon,pfi/pficommon,retrieva/pficommon,takei-yuya/pficommon,retrieva/pficommon,takei-yuya/pficommon,pfi/pficommon,pfi/pficommon