When checking loop bodies and do-expr bodies, don't require the expected type to exist

If the expected type is none (due to a type error), we shouldn't fail with an ICE, but
rather, just print out another type error. Changed the do-expr type error message to
make sense in this context (see the test case for how it works).

Closes #3044.

src/test/compile-fail/issue-3044.rs

@@ -0,0 +1,9 @@
+// error-pattern: Non-function passed to a `do` function as its last argument, or wrong number of arguments passed to a `do` function
+fn main() {
+    let needlesArr: ~[char] = ~['a', 'f'];
+    do vec::foldr(needlesArr) |x, y| {
+    }
+// for some reason if I use the new error syntax for the two error messages this generates,
+// the test runner gets confused -- tjc
+}
+