fix(transfer-dag): add needed assume role and stop discarding exceptions

[anayeaye]

What

• Raise exception at end of transfer loop if any exceptions or client errors occurred during loop
• Correct format of variable stored in aws secrets manager (string required)
• Add assume role arns to airflow_custom_variables stored in secrets manager for DAGs

Why

• Transfer DAG successfully completed without reporting client errors
• Assume role arns were needed for successful boto copy in transfer dag

How tested

• Deployed to smce-staging and executed transfer DAG
• Confirmed that assume role arns were added to airflow custom variables in secrets manager