Handle prediction of uncompleted Autopilot job

[umitbuyuksahin]

• if a job is interrupted before completion due to working of one of the "max_runtime" stopping criteria, autopilot might have several candidate but might not have an best candidate.
• we can throw an exception saying that there is not a best candidate.

[exa-eswar]

"max_runtime" is one of the criteria to not select the best candidate. It would be helpful for user to understand what to do inorder to get best candidate. e.g.: If is it max_runtime error, throwing an exception to increase the max_runtime, also showing the current max_runtime would be helpful.