gen_isr_tables: Improve error message

Users are reportedly not able to understand how to debug the following
error message from gen_isr_tables:

gen_isr_tables.py: multiple registrations at table_index 8 for irq 8

Debugging issues these kinds of issues is difficult so we need to give
users as much information as possible.

To make it clearer that it could be an abuse of the 'IRQ_CONNECT' API
that is causing the issue we add this to the error message.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>