Refactor: tools: Functionize danger checks in crm_shadow

Create a function set_danger_error() to avoid repetition of the "To
prevent accidental destruction..." string.

Also previously, if we returned after finding the instance mismatch,
then we never warned of the inherent danger of the command (that is,
overwriting the active cluster or deleting the shadow file). It seems
more appropriate to warn of the inherent danger no matter what, and warn
of the mismatch if appropriate.

Signed-off-by: Reid Wahl <nrwahl@protonmail.com>