Add an intermediate 'warning' state

[RoryO]

As of right now we either have a total success or failure state. Sometimes we are processing a batch of records where, for example, some may not pass validations for unrelated reasons. We don't want to signify that the entire run was a failure, but instead collect warnings that will displayed and cataloged as success messages.

To support this, we need:

• Schema change to add a warnings column in the schlepper_tasks table
• Check the schema definition on process run to determine if there are schema changes that need to be applied
• Generate a @warnings array ivar in a new schlepper task script
• Document @warnings usage in the generated class and in the usage README
• Check for run.respond_to?(:warnings) && run.warnings.present after the run completes
• If there are warnings for the run
  • Pretty print the warnings to the console
  • Dump the warnings into the success row for the run