Improve nonexistent-module error message

The error message for (for example) "import vec;" without "use std;"
was "cyclic import", which was misleading because there were no
cycles. I changed it to "cyclic import or nonexistent module",
which doesn't break existing tests.

src/comp/middle/resolve.rs

@@ -724,7 +724,7 @@ fn lookup_import(&env e, def_id defid, namespace ns) -> option::t[def] {
             ret lookup_import(e, defid, ns);
         }
         case (resolving(?sp)) {
-            e.sess.span_err(sp, "cyclic import");
+            e.sess.span_err(sp, "cyclic import or nonexistent module");
         }
         case (resolved(?val, ?typ, ?md)) {
             ret alt (ns) { case (ns_value) { val }

src/test/compile-fail/bad-module.rs

@@ -0,0 +1,7 @@
+// xfail-stage0
+// error-pattern: nonexistent module
+import vec;
+
+fn main() {
+  auto foo = vec.len([]);
+}