Handling resource assignment failure

[bozhiyou]

https://github.com/ut-parla/Parla.py/blob/master/parla/task_runtime.py#L790-L795
It warns when the assignment fails too many times: Failed to assign devices. The required resources may not be available on this machine at all. However, cases can be that tasks are competing for a particular device, e.g. many tasks wanting a particular GPU. The failure should be warned with more details and there may be another way to handle the competing case.

[arthurp]

Duplicates #29