west: runners: core.py: Remove no-op try-excepts

Removing these doesn't change behavior, since the
subprocess.CalledProcessError is just immediately re-raised when caught.

Fixes this pylint warning:

    W0706: The except handler raises immediately (try-except-raise)

Fixing pylint warnings for a CI check.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>