scripts: runners: bossac: handle edtlib ImportError

Runners may not import anything outside of stdlib unless they handle
the possible ImportError.

Apply this rule to the bossac runner. The general pattern is to have a
MISSING_REQUIREMENTS global that we handle at the beginning of
do_run(), but in this case this isn't a missing pip package, but a
misconfiguration. Just log a warning; it won't happen in practice.

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