Kconfig: STACK_CANARIES: Correct the help text

The help text has been stating that CONFIG_STACK_CANARIES will
silently be ignored when the compiler does not support them. But this
is not the desired behaviour of CONFIG_STACK_CANARIES[1].

This patch corrects the help text to state that an error will occur if
this feature is enabled, but not supported.

[1] "I would much rather see the build break if someone tries to
enable the stack canaries, and the compiler doesn't support
it. Because what happens now is that if someone enables this option,
and there is no support, the build will succeed but there are no
actual stack canaries in place, and unless the user is paying close
attention to the cmake test output they will have no idea."
--
https://github.com/zephyrproject-rtos/zephyr/issues/5019



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>