Better error for tuple variable declarations with non-tuple type expression

[benharsh]

Consider the following program:

proc main() {
  var a = (1, 2, 3);
  var (x,y,z): int = a;
}

This gives the error:

baz.chpl:1: In function 'main':
baz.chpl:3: error: illegal tuple variable declaration with non-tuple initializer

Personally, I find this error message to be rather unhelpful as it suggests something about the initialization expression is wrong, rather than the type expression. I initially wrote this with multi-decl rules in mind, and thought I was specifying the element type of each tuple component. So when I first saw this error, I thought it might be a compiler bug with normalization or something rather than doing the wrong thing myself.

Changing int to 3*int gets things working, and it would be nice if the error message could suggest something along these lines.