[FR] Error messages should be more specific

Users at PROBPROG 2018 requested more-specific error messages when models are invalid.

Specific improvements include:

• #1509 Print sample site information when Distribution inputs are invalid. Currently it requires a debugger to see what line of the model is affected.
• #1534 On error print shapes of sites in the trace: value shape for params, and value,dist,log_prob shapes for sample sites.
• On error, print contextual information: the current cond_indep_stack, the portion of stack trace under svi.step(), whether the model or guide is being executed, _DIM_ALLOCATOR sate, tracing state, etc. This could be accomplished via a try: except: raise block to add information to a caught error.
• others?