Do not abort compilation if expansion produces errors

Fix a number of uncovered deficiencies in diagnostics

src/libsyntax_ext/format.rs

@@ -666,7 +666,7 @@ impl<'a, 'b> Context<'a, 'b> {
                     "X" => "UpperHex",
                     _ => {
                         ecx.span_err(sp, &format!("unknown format trait `{}`", *tyname));
-                        "Dummy"
+                        return DummyResult::raw_expr(sp);
                     }
                 }
             }
@@ -713,7 +713,7 @@ pub fn expand_format_args_nl<'cx>(
                                        sp,
                                        feature_gate::GateIssue::Language,
                                        feature_gate::EXPLAIN_FORMAT_ARGS_NL);
-        return base::DummyResult::expr(sp);
+        return DummyResult::expr(sp);
     }
     sp = sp.apply_mark(ecx.current_expansion.mark);
     match parse_args(ecx, sp, tts) {