Check that a relative name plus the zone's origin is not too long. (#997)

Check that a relative name plus the zone's origin is not too long. (#997)

Previously it was possible to add very long relative names to a
relative zone which could never be rendered due to being too long for
wire format.  Now we check this as part of _validate_name().

This code also removes duplicated name validation code from Zone and
Version, consolidating it into one helper function.

Finally, we fix a few comments in get methods that have cut-and-paste
typos from the find variant indicating they can raise KeyError when
they cannot.