Report better errors for packs that are not valid zip files.
Report better errors for packs that are not valid zip files.

mbedmicro/pyOCD,flit/pyOCD,mbedmicro/pyOCD,pyocd/pyOCD,pyocd/pyOCD,flit/pyOCD,mesheven/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD,mesheven/pyOCD