Fix error message on FILE_TYPE_MISMATCH

Previously the error message reported the compute type internal to the
reference model as not matching, but the relevant dtype is the
serialization storage dtype. These two match in all cases except the
--precise_mode=1 reference model with floating point dtype.

Signed-off-by: Ian Tayler Lessa <ian.taylerlessa@arm.com>
Change-Id: I08e1e2cf5fa95b242d870af9364e635dda254c0e