Clarify meaning of is_unreachable

[tomaka]

Close #1531
cc #2746

This PR renames is_unreachable to is_bad_address.
It is generally not possible to know whether an address is definitely unreachable or not. The only situation where we can affirm that an address is definitely unreachable is if our implementation doesn't support a certain protocol that it uses.

This matches what the browser node already does anyway.

[mergify]

Automatically approving tomaka's pull requests. This auto-approval will be removed once more maintainers are active.

[tomaka]

For what it's worth, I don't really like the API of pending_outcome_err as it is a bit too vague/magic. This is covered by #2746

[github-actions]

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