Added a flag to HivePartitionTarget to prevent failure if a table doesn't exist

This enables the following dependency graph:

{Generate Data} -> {Generate Schema} -> {Create Table} -> {Add Partition}

The Add Partition task has an output type of HivePartitionTarget, however the DAG doesn't execute (errors) if the table isn't created before that target is evaluated.

My change allows you to override this behavior and return 'False' instead of raising an exception.