Assignability error reporting should have a more specific error for unions

[sandersn]

interface A {
    a: number
}
interface B {
    b: number
}
interface C {
    c: number
}
interface D {
    d: number
}
declare var c: C;
let abc: ((A | B) & C) | D = c;

Actual error:

Type 'C' is not assignable to type '((A | B) & C) | D'.
  Type 'C' is not assignable to type 'D'.
    ....

Expected error:

Type 'C' is not assignable to any branch of the union type '((A | B) & C) | D'.
  For example, type 'C' is not assignable to type 'D'.
    ....

Error reporting on unions doesn't mention that the compiler tried to assign the source to all branches of the target. Instead it proceeds to elaborate just the last branch's error. It should make it clear that the error elaboration is just one of several checks that failed.

Otherwise it is easy to assume that the compiler only tried the last branch and somehow skipped the "more promising" branch (which ultimately also fails, but at least contains the source type).

Based on one of the multiple issues reported by @Aleksey-Bykov in #6538.