Improve errors for use-before-init within init() routines

[bradcray]

In a recent SO question, a user expressed confusion over getting a "use before init" error for a case like this:

record R {
  var x: [1..3] int;
  proc init() {
    for i in 1..3 do x[i] = i;  // error: field "x" used before it is initialized
  }
}

var myR: R;

which is understandable since it appears as though the line in question is initializing the array rather than using it, logically speaking. Using --detailed-errors didn't seem to improve the message as of Chapel 2.4.

Here, I'm wondering whether we could specialize this error message for the initializer context to add some additional detail like "Within an initializer, each field must be initialized using an assignment of the form [this.]field = expr; before it is referenced." We could potentially add "Or use an init this; statement before referencing the field to opt into default field initialization."