Cleanup app error codes

1. The error classes (like AppsError) now take a 3rd argument details.
We can attach anything in this 3rd argument and this gets sent in the
REST response as well.

2. The HttpError class is now HttpError(statusCode, errorOrMessage). If
it's an error object, it will take the message and other things which
were attached above from it and send them across. Previously, we used to
mark this case an internal error all the time.

3. AppsError only has generic codes now. The UI code then simply checks
for additional information that we attached to show errors. For example,
BAD_FIELD will have a field: 'xx' indicating which field is at fault.
ALREADY_EXISTS has information on which domain or port caused a problem.
The advantage here is we can drop all these error codes that are
specific to each model code.

4. Maybe some day, we can remove all these error classes and have only
one generic class. AppsError right now is pretty generic already. We can
use that error code everywhere... No need to translate errors also
everywhere.

5. Finally, in the router code, I have this function toHttpError (in
apps.js) which is also so much cleaner than what we have now. We keep
writing the same stuff over and over.

src/routes/apps.js

@@ -41,12 +41,9 @@ function toHttpError(appError) {
     case AppsError.NOT_FOUND:
         return new HttpError(404, appError);
     case AppsError.ALREADY_EXISTS:
-    case AppsError.PORT_CONFLICT:
-    case AppsError.LOCATION_CONFLICT:
     case AppsError.BAD_STATE:
         return new HttpError(409, appError);
     case AppsError.BAD_FIELD:
-    case AppsError.BAD_CERTIFICATE:
         return new HttpError(400, appError);
     case AppsError.PLAN_LIMIT:
         return new HttpError(402, appError);