flash/nor/nrf5: improve handling of nRF52 flash errors

flash/nor/nrf5: improve handling of nRF52 flash errors

nRF52 devices indicate a flash error by emitting hard fault
exception (unlike nRF51 series).

Change error message when NVMC READY read fails. A hard fault
from flash erase/write operation is detected here.

Check exit point of the flash write algo to ensure a failed
write is recognised.

Change-Id: I637eda268a6bf45f7f41bcb9dcd82db8f5cb41b4
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5587
Tested-by: jenkins