mem: make MemTest panic on a packet error

mem: make MemTest panic on a packet error

Before this change, running:

./build/NULL/gem5.opt configs/example/ruby_mem_test.py -m 20000000 \
  --functional 10

would only print warning for memory errors such as:

warn: Read access failed at 0x107a00

and there was no way to make the simulation fail.

This commit makes those warnings into errors such as:

panic: Read access failed at 0x107a00

unless --suppress-func-errors is given.

This will be used to automate MemTest testing in later commits.

Change-Id: I1840c1ed1853f1a71ec73bd50cadaac095794f91
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26804
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>