Ignore missing tags when using --skip-tags

The rationale behind this change is that I don't really care if a tag is
missing when I want to skip it.

One potential reason to prefer to throw an error is to prevent a user
from executing a tag that they did not intend to execute as theoretically
that could cause issues. But in practice, one should not rely on
--skip-tags for avoiding disastrous results. There are better ways to
not break your hosts.

Alternatively, I can add an option, say --ignore-missing-tags, that will
instruct --skip-tags not to throw an error.