Auto merge of #858 - fitzgen:report-unknown-generate-item, r=emilio

Report what the unknown generate item was when parsing `--generate`

r? @emilio

Author: bors-servo

src/options.rs

@@ -302,9 +302,10 @@ pub fn builder_from_flags<I>
                 "methods" => config.methods = true,
                 "constructors" => config.constructors = true,
                 "destructors" => config.destructors = true,
-                _ => {
+                otherwise => {
                     return Err(Error::new(ErrorKind::Other,
-                                          "Unknown generate item"));
+                                          format!("Unknown generate item: {}",
+                                                  otherwise)));
                 }
             }
         }