Fix cert test crashes when running on remote machine

When cert test is executed on a remote machine, against locally
connected devices with ports forwarded to remote machine,
crashes are observed during cleanup that happens in setup.

This is for cleanup of forwarded ports, when no forwarding is setup, or
for removal of btsnoop.log when it's not present.

After this patch, non-fatal errors would print error message rather than
crash the whole test.

Tag: #gd-refactor
Test: gd/cert/run LeSecurityTest
Change-Id: I68df152ba83a416cb81b26f31ae9ebe3698e3f02