Print the diff when "./gradlew verGJF" fails in CI.

[sebright2]

See https://github.com/census-instrumentation/opencensus-java/pull/1141#issuecomment-388479392. When ~~google-java-format~~ google-java-format-gradle-plugin fails, it lists the files with incorrect formatting, but it doesn't describe the error. We could provide more information about the failure by running "./gradlew goJF" and then printing the diff when the command fails in CI.

Related google-java-format-gradle-plugin issue: https://github.com/sherter/google-java-format-gradle-plugin/issues/19