arm: cortex-m: enhance information dump during HardFault escalation

When inside an escalated HardFault, we would like to get
more information about the reason for this escalation. We
first check if the reason for thise escalation is an SVC,
which occurs within a priority level that does not allow
it to trigger (e.g. fault or another SVC). If this is true
we set the error reason according to the provided argument.

Only when this is not a synchronous SVC that caused the HF,
do we check the other reasons for HF escalation (e.g. a BF
inside a previous BF).

We also add a case for a debug event, to complete going through
the available flags in HFSR.

Finally we ASSERT if we cannot find the reason for the escalation.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>