scripts: runners: error on missing non-elf outputs

The RunnerConfig class stores the locations of the Zephyr output files
in various formats (elf, hex, bin). A longstanding issue with the
representation is that these might not exist if the corresponding
Kconfig options are not set. For example, if
CONFIG_BUILD_OUTPUT_BIN=n, there is no .bin file.

Change this so the type system knows these are Optional[str], not str.

Fix the runners that use non-ELF outputs so they check for the
existence of the relevant file before using it, mostly using a new
ZephyrBinaryRunner.ensure_output helper.

I'm not going to bother with checking for the ELF file itself; that's
always there as far as I can tell.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>