Let transports fail gracefully

[rumpelsepp]

If e.g. a non existent can interface is specified, gallia fails because dumpcap fails with a non zero exit code. The reason for this is that dumpcap is called before load_transport()

[rumpelsepp]

This case must be handled better.

$ gallia scan uds identifiers --target "foobar"

[stale]

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