rework Metropolis rejection warning

[bob-carpenter]

We're torn between wanting to leave the reject warnings in to help users diagnose errors and getting rid of them to stop users from freaking out when seeing them.

We also want them to be more informative.

Some specifics:

• Fix error messages to clarify size mismatches and also hint that they may be the cause of rejection.

  Informational Message: The parameter state is about to be Metropolis
  rejected due to the following underlying, non-fatal (really) issue 
  

  for which I suggest adding

  check that all functions and distributions have consistently sized,
  in-support arguments 
  

• Fix

  Error in function stan::prob::normal_log(m): Location parameter (max size)
  is 256, but must be consistent, 1 or max=768 
  

  which could be changed to

  a vectorized function was called with arguments of different scalar, array, vector,
  or matrix types, and they were not consistently sized;  all arguments
  must be scalars or multidimensional values of the same shape;  found dimensions=
  <list of argument shapes passed in> 
  

[syclik]

I'm creating a pull request for the second checkbox. I don't think we should do the first -- I think we should be throwing a different type of error that will stop sampling if the sizes are inconsistent.