detect unused attrs in one more place, allow parsing to continue for all

changed a bunch of fatal()'s into err()'s, to allow parsing to proceed.

src/test/compile-fail/attr-before-ext.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 fn main() {
-    #[attr]
-    debug!("hi"); //~ ERROR expected item after attrs
+    #[attr] //~ ERROR expected item after attributes
+    debug!("hi");
 }

src/test/compile-fail/attr-before-let.rs

@@ -9,6 +9,6 @@
 // except according to those terms.
 
 fn main() {
-    #[attr]
-    let _i = 0; //~ ERROR expected item
+    #[attr] //~ ERROR expected item
+    let _i = 0;
 }