Fail if ~/.m2/settings.xml is not present

[TheSnoozer]

From #12 (comment).
GPG fails with cryptic errors when ~/.m2/settings.xml is not present.

Perhaps can we update the dependency to fix this?

[TheSnoozer]

Added it to the release-checklist, feels silly to implement any logic in the plugin(s). This should really be part of the releasing plugin or something. Anyhow a TODO note feels sufficient.