tc-testing: tdc.py: ignore errors when decoding stdout/stderr

Prevent exceptions from being raised while decoding output
from an executed command. There is no impact on tdc's
execution and the verify command phase would fail the pattern
match.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>