Produce better errors for invalid imports.

src/Makefile

@@ -404,6 +404,7 @@ TEST_XFAILS_BOOT :=  $(TASK_XFAILS) \
                     test/run-fail/task-comm-14.rs \
                     test/compile-fail/import.rs \
                     test/compile-fail/import2.rs \
+                    test/compile-fail/import3.rs \
                     test/compile-fail/bad-recv.rs \
                     test/compile-fail/bad-send.rs \
                     test/compile-fail/infinite-vec-type-recursion.rs \

src/comp/middle/resolve.rs

@@ -90,6 +90,12 @@ fn find_final_def(&env e, &span sp, vec[ident] idents) -> option.t[def_wrap] {
                     auto new_e = update_env_for_item(tmp_e, i);
                     ret find_final_def(new_e, sp, new_idents);
                 }
+                case (def_wrap_use(?c)) {
+                    e.sess.span_err(sp, "Crate access is not implemented");
+                }
+                case (_) {
+                    e.sess.span_err(sp, first + " is not a module or crate");
+                }
             }
         }
     }

src/test/compile-fail/import3.rs

@@ -0,0 +1,6 @@
+// error-pattern: main is not a module or crate
+import main.bar;
+
+fn main(vec[str] args) {
+  log "foo";
+}