target/arm: Handle page table walk load failures correctly

Instead of ignoring the response from address_space_ld*()
(indicating an attempt to read a page table descriptor from
an invalid physical address), use it to report the failure
correctly.

Since this is another couple of locations where we need to
decide the value of the ARMMMUFaultInfo ea bit based on a
MemTxResult, we factor out that operation into a helper
function.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
