Consolidate code for json schema validation

[jeffbl]

All of the preprocessors (and likely handlers) repeat the same boilerplate code for all of the json schema checking. This is error-prone and bloats up the size of the preprocessors. This code should be consolidated into helper functions, or via some other method, so that simply specifying the schemas to be used is enough. This would also help with having to refactor in the future, e.g., for issue #912, where we want to change the schema error logging for all preprocessors and handlers.

• create method for doing this
• update all preprocessors/handlers to use this new code
• make sure that builds are kicked off appropriately if this consolidated code is changed

Note that other such simplifications may present themselves. Anything we can do to avoid repeating the same code in multiple places should make it easier for developers to create and maintain IMAGE handlers and preprocessors. These should be logged as separate issues for discussion.