aarch64: Add an error code for out-of-range registers

aarch64: Add an error code for out-of-range registers

libopcodes currently reports out-of-range registers as a general
AARCH64_OPDE_OTHER_ERROR.  However, this means that each register
range needs its own hard-coded string, which is a bit cumbersome
if the range is determined programmatically.  This patch therefore
adds a dedicated error type for out-of-range errors.