Use convenience methods to check for reply type

It is often necessary to check if a reply is of a specific type. This
involves checking the type of the reply object against a class object.
This can be tedious to do and can suitably be wrapped in convenience
methods in the reply class. We already have such a method for OK status
replies. Add similar methods for other types of replies that we check
for and use them instead.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>