distribution variable names in error messages to match manual

[bob-carpenter]

Change our error messages to describe variables using the descriptive name and the variable name from the manual, as in "scale argument sigma" or "location argument mu"

We don't want to use "parameter" because it can be confused with the parameters in a Stan program. And names like "precision" can be confused with numerical precision (thanks to https://github.com/jeffalstott for pointing this out and suggesting the new form in #297; this issue is more general and calls for a review of all of our distribution functions).

[syclik]

I was just looking at this. Should we just refer to things as their argument names? That would be pretty easy to fix and make the manual match.

For example, for the normal distribution:

• y instead of random variable
• mu instead of location parameter
• sigma instead of scale parameter
  ?