Slightly more helpful error message for "null lib handle"

I changed the error message to also suggest checking the -L flag
when this happens.

src/rt/rust_crate_cache.cpp

@@ -49,7 +49,8 @@ rust_crate_cache::c_sym::c_sym(rust_dom *dom, lib *library, char const *name)
         DLOG(dom, cache, "resolved symbol '%s' to 0x%"  PRIxPTR,
                  name, val);
     } else {
-        DLOG_ERR(dom, cache, "unresolved symbol '%s', null lib handle", name);
+        DLOG_ERR(dom, cache, "unresolved symbol '%s', null lib handle\n"
+                             "(did you omit a -L flag?)", name);
     }
 }