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Issue Description:

The error message "Switch must exhaustive, consider adding a default clause" should be improved by stating at least one case that is incompletely handled so the programmer can immediately do something to start fixing the problem.

Consider the following code in Swift 3.1, which will give the error "Switch must exhaustive, consider adding a default clause":

enum Something {
case a, b, c, d, r, e, f, g
}

let s: Something = someFunctionReturningSomething()
switch s {
case .a: print("a")
case .b: print("b")
case .c: print("c")
case .d: print("d")
case .e: print("e")
case .f: print("f")
case .g: print("g")
}

Upon seeing the error message, you can't immediately fix the problem. Instead, you need to slowly and painstakingly analyze the code, comparing cases in the enum to cases in the switch to find which case is accidentally omitted. This can be difficult in the presence of where clauses and can be very time consuming for enums with a large number of cases.

This is work that the compiler should be able to do (theoretically, it has already done the work to recognize the need for the error in the first place).

A more helpful message would be: "Switch must be exhaustive; cases, including 'r', are incompletely handled. Consider adding a default clause."