scripts: runners: handle failure to import individual runners

We need all available runners to be defined as subclasses of
runners.core.ZephyrBinaryRunner in order to be able to look them up by
name at runtime. We do this by importing them from runners.__init__.

This process periodically fails when some runner or other cannot be
imported, usually because it is trying to import something outside of
stdlib and not handling ImportError.

Rather than letting this bring down the entire Python process, catch
and log the error. Sort the list again while we're here.

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