Mass rename if_ok! to try!

This "bubble up an error" macro was originally named if_ok! in order to get it
landed, but after the fact it was discovered that this name is not exactly
desirable.

The name `if_ok!` isn't immediately clear that is has much to do with error
handling, and it doesn't look fantastic in all contexts (if if_ok!(...) {}). In
general, the agreed opinion about `if_ok!` is that is came in as subpar.

The name `try!` is more invocative of error handling, it's shorter by 2 letters,
and it looks fitting in almost all circumstances. One concern about the word
`try!` is that it's too invocative of exceptions, but the belief is that this
will be overcome with documentation and examples.

Close #12037

src/librustdoc/lib.rs

@@ -351,7 +351,7 @@ fn json_output(krate: clean::Crate, res: ~[plugins::PluginJson],
     json.insert(~"crate", crate_json);
     json.insert(~"plugins", json::Object(plugins_json));
 
-    let mut file = if_ok!(File::create(&dst));
-    if_ok!(json::Object(json).to_writer(&mut file));
+    let mut file = try!(File::create(&dst));
+    try!(json::Object(json).to_writer(&mut file));
     Ok(())
 }