lang: unification: Catch unification error on typed if expr

I found a case where we had two missing unification rules. Now fixed in
the previous commits, and including this test to show I'm responsible.
I've added the same test in two locations for redundancy and as an
example.

lang/interpret_test/TestAstFunc1/fail2/main.mcl

@@ -0,0 +1,9 @@
+import "fmt"
+$x str = if true {	# should fail unification
+	42
+} else {
+	13
+}
+test "t1" {
+	anotherstr => fmt.printf("hello %s", $x),
+}