Add more sanity checks around callers of changeDependencyFor()

Hi all,

While working on a different patch, I have noted three code paths that
call changeDependencyFor() but don't check that they do not return
errors.  In all the three cases (support function, extension/schema
and object/schema), it seems to me that only one dependency update is
expected.

I am adding that to the next CF.  Thoughts or comments about the
attached?
--
Michael