valueFromContext(ctx, errors, id, args) {
    const entity = ctx.messages.get(id);

    if (entity === undefined) {
      errors.push(new L10nError(`Unknown entity: ${id}`));
      return id;
    }

    return ctx.format(entity, args, errors);
  }
Format the value of a message into a string.

This function is passed as a method to `keysFromContext` and resolve
a value of a single L10n Entity using provided `MessageContext`.

If the function fails to retrieve the entity, it will return an ID of it.
If formatting fails, it will return a partially resolved entity.

In both cases, an error is being added to the errors array.

Subclasses of `Localization` can override this method to provide
environment-specific formatting behavior.

@param   {MessageContext} ctx
@param   {Array<Error>}   errors
@param   {string}         id
@param   {Object}         args
@returns {string}
@private