[Repo::Push] When validator raises, show the underlying exception message.

Refs https://github.com/CocoaPods/CocoaPods/issues/3108.

lib/cocoapods/command/repo/push.rb

@@ -86,8 +86,9 @@ module Pod
             validator.allow_warnings = @allow_warnings
             begin
               validator.validate
-            rescue
-              raise Informative, "The `#{podspec}` specification does not validate."
+            rescue => e
+              raise Informative, "The `#{podspec}` specification does not validate." \
+                                 "\n\n#{e.message}\n\n#{e.backtrace * "\n"}"
             end
             raise Informative, "The `#{podspec}` specification does not validate." unless validator.validated?
           end