Ignore missing commands when ignoring errors

[cdown]

Commands which fail, even at (for example) dynamic loader stage get a pass if there's a "-" leader to indicate to ignore errors. That means that any user using this must accept that there's a chance that the command will not execute whatsoever.

Since these outcomes are the same as the executable not existing at all, we should allow "-" to also silence the command not existing in the first place.