Introduce a T_err type for type errors

This allows more errors to be non-fatal, as per #1871.

I only went through and started changing span_fatal to span_err in
check.rs. There are probably more errors that could be made
non-fatal. So if you see derived type errors appearing from now on,
file a bug!

r=graydon

Closes #1871

src/test/compile-fail/cast-from-nil.rs

@@ -1,2 +1,2 @@
-// error-pattern: cast from nil: () as u32
+// error-pattern: cast from nil: `()` as `u32`
 fn main() { let u = (assert true) as u32; }

src/test/compile-fail/cast-to-nil.rs

@@ -1,2 +1,2 @@
-// error-pattern: cast to nil: u32 as ()
+// error-pattern: cast to nil: `u32` as `()`
 fn main() { let u = 0u32 as (); }

src/test/compile-fail/extern-no-call.rs

@@ -1,4 +1,4 @@
-// error-pattern:expected function or foreign function but found *u8
+// error-pattern:expected function or foreign function but found `*u8`
 extern fn f() {
 }
 

src/test/compile-fail/issue-1871.rs

@@ -1,11 +1,12 @@
-// xfail-test
+// Tests that we don't generate a spurious error about f.honk's type
+// being undeterminable
 fn main() {
   let f = 42;
 
   let _g = if f < 5 {
-      f.honk();
+      f.honk() //~ ERROR attempted access of field `honk`
   }
   else {
-    12
+      ()
   };
 }

src/test/compile-fail/issue-2149.rs

@@ -11,5 +11,5 @@ impl<A> ~[A]: vec_monad<A> {
    }
 }
 fn main() {
-    ["hi"].bind({|x| [x] });
+    ["hi"].bind({|x| [x] }); //~ ERROR attempted access of field `bind`
 }