riscv: clarify error message on undefined exceptions

Undefined exceptions are treated as reserved. This is not clearly
communicated to the user. Adjust the error message to clarify that a
reserved exception has occurred and add additional details.

Fixes: e8b522b1 ("riscv: treat undefined exception codes as reserved")
Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>