core: EvalSequence to handle EvalEarlyExitError

An earlier commit today reworked this to handle non-fatal errors, which
are returned "smuggled" as a special type of error to avoid changing the
EvalNode interface.

Unfortunately, that change then broke the _other_ special thing we smuggle
through the error return path: early exit.

Now we'll handle them both. This is not perfect because the early-exit
path causes us to discard any warnings we've already collected, but it's
more important that we bail early than retain warnings.

terraform/eval_sequence.go

@@ -18,6 +18,11 @@ func (n *EvalSequence) Eval(ctx EvalContext) (interface{}, error) {
 		}
 
 		if _, err := EvalRaw(n, ctx); err != nil {
+			if _, isEarlyExit := err.(EvalEarlyExitError); isEarlyExit {
+				// In this path we abort early, losing any non-error
+				// diagnostics we saw earlier.
+				return nil, err
+			}
 			diags = diags.Append(err)
 			if diags.HasErrors() {
 				// Halt if we get some errors, but warnings are okay.