Do not abort compilation if expansion produces errors

Fix a number of uncovered deficiencies in diagnostics

src/test/ui/proc-macro/proc-macro-attributes.rs

@@ -3,11 +3,12 @@
 #[macro_use]
 extern crate derive_b;
 
-#[B]
+#[B] //~ ERROR `B` is ambiguous
 #[C] //~ ERROR attribute `C` is currently unknown to the compiler
-#[B(D)]
-#[B(E = "foo")]
-#[B(arbitrary tokens)]
+#[B(D)] //~ ERROR `B` is ambiguous
+#[B(E = "foo")] //~ ERROR `B` is ambiguous
+#[B(arbitrary tokens)] //~ ERROR `B` is ambiguous
+                       //~^ ERROR expected one of `(`, `)`, `,`, `::`, or `=`, found `tokens`
 #[derive(B)]
 struct B;