* lib/rubygems/security/policy.rb:  Raise proper exceptions when
  verifying unsigned gems (instead of crashing).
* test/rubygems/test_gem_security_policy.rb:  Tests for the above.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39153 b2dd03c8-39d4-4d8f-98ff-823fe69b080e