Specify exception types for QobjEvo
Specify exception types for QobjEvo

Several exceptions in QobjEvo were just raising Exception rather than
raising a specific error.  This made it more difficult for user code to
catch allowable failings in their own code, and made it harder to
determine what the problem had been.

cgranade/qutip,cgranade/qutip,qutip/qutip,qutip/qutip