Print out a more helpful type error message for do-blocks/for-loops

If a do-block body has the wrong type, or a for-loop body has a
non-() type, suggest that the user might have meant the other one.

Closes #2817

r=brson

src/test/compile-fail/block-must-not-have-result-for.rs

@@ -1,7 +1,5 @@
-// error-pattern:mismatched types: expected `()` but found `bool`
-
 fn main() {
-    for vec::each(~[0]) |_i| {
+    for vec::each(~[0]) |_i| {  //~ ERROR A for-loop body must return (), but
         true
     }
 }

src/test/compile-fail/issue-2817-2.rs

@@ -0,0 +1,16 @@
+fn not_bool(f: fn(int) -> ~str) {}
+
+fn main() {
+    for uint::range(0, 100000) |_i| { //~ ERROR A for-loop body must return (), but
+        false
+    };
+    for not_bool |_i| { //~ ERROR a `loop` function's last argument should return `bool`
+        //~^ ERROR A for-loop body must return (), but
+        ~"hi"
+    };
+    for uint::range(0, 100000) |_i| { //~ ERROR A for-loop body must return (), but
+        ~"hi"
+    };
+    for not_bool() |_i| { //~ ERROR a `loop` function's last argument
+    };
+}

src/test/compile-fail/issue-2817.rs

@@ -0,0 +1,15 @@
+fn uuid() -> uint { fail; }
+
+fn from_str(s: ~str) -> uint { fail; }
+fn to_str(u: uint) -> ~str { fail; }
+fn uuid_random() -> uint { fail; }
+
+fn main() {
+    do uint::range(0, 100000) |_i| { //~ ERROR Do-block body must return bool, but
+    }
+    // should get a more general message if the callback
+    // doesn't return nil
+    do uint::range(0, 100000) |_i| { //~ ERROR mismatched types
+        ~"str"
+    }
+}