armv7m: Explicit error for bad vector table

Give an explicit error and abort when a load
from the vector table fails. Architecturally this
should HardFault (which will then immediately
fail to load the HardFault vector and go into Lockup).
Since we don't model Lockup, just report this guest
error via cpu_abort(). This is more helpful than the
previous behaviour of reading a zero, which is the
address of the reset stack pointer and not a sensible
location to jump to.

Signed-off-by: Michael Davidsaver <mdavidsaver@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 1484937883-1068-4-git-send-email-peter.maydell@linaro.org
[PMM: expanded commit message]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>