Replace DBError with DeletionFailed for DataSource and Job

Foreign key constraints on DataSource and Job objects by JobExecutions
cause DBError exceptions if those objects are deleted while they are
referenced by a JobExecution.  Replace the DBerror with a SavannaException
so that the REST api reports the error nicely.

Change-Id: I756f561b019895c8c2c39f17bd3a05da31ad6440
Closes-Bug: #1235229
Merge "Replace DBError with DeletionFailed for DataSource and Job"