sim: Add some helpers to catch and reporting using unbound ports.

sim: Add some helpers to catch and reporting using unbound ports.

If a port is unbound, trying to call its peer will likely cause a
segfault. Rather than check if a port is bound every time you go to use
it, we can instead bind to a default peer which just throws an exception
back to the caller. The caller can catch the exception and report the
error.

This change adds a common new class to throw as the exception, and also
a small utility function which reports the error and dies.

Change-Id: Ia58a2030922c73e2fd7d139822bce38d9b0f2171
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/30295
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>