In situations where a user of the CmdResult class manually sets the
stdout/stderr attributes to a string, instead of the recommended and
documented content of type "bytes", it won't be possible to "decode"
the bytes into a string of a given encoding, and we'll end up with
a crash.

Since the goal of std{out,err}_text is to return a text version
of stdout, if itself already is of such a type, let's just return
it.

Additionally, if the data cannot be "decode()d", let's raise an
explicit TypeError on this location, rather than later in the code
when its value is attempted to be used.
Signed-off-by: NCleber Rosa <crosa@redhat.com>