changed error handling in retrieval_history model, added custom error to source model.
changed error handling in retrieval_history model, added custom error to source model.

ankurpohekar/Lagotto,articlemetrics/lagotto,lagotto/lagotto,msingh05/lagotto,mfenner/lagotto,mfenner/lagotto,mfenner/lagotto,articlemetrics/lagotto,lagotto/lagotto,articlemetrics/lagotto,CrossRef/lagotto,lagotto/lagotto,ankurpohekar/Lagotto,CrossRef/lagotto,ankurpohekar/Lagotto,msingh05/lagotto,ankurpohekar/Lagotto,ankurpohekar/Lagotto,CrossRef/lagotto,ankurpohekar/Lagotto,msingh05/lagotto,ankurpohekar/Lagotto