Fix exception when core type cannot be read
Fix exception when core type cannot be read

Log "unknown" for core type when it cannot be read rather than raising
a KeyError. This makes it possible to run scripts on locked ST targets.

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