* Exit if loading an invalid identity from disk
Previously, if an invalid identity was loaded from disk, ZeroTier would
generate a new identity & chug along and generate a brand new identity
as if nothing happened. When running in containers, this introduces the
possibility for key matter loss; especially when running in containers
where the identity files are mounted in the container read only. In
this case, ZT will continue chugging along with a brand new identity
with no possibility of recovering the private key.
ZeroTier should exit upon loading of invalid identity.public/identity.secret #2056
* add validation test for #2056
