Everywhere we print an fxp_error(), try to make it clear what we were trying

Everywhere we print an fxp_error(), try to make it clear what we were trying
to do at the time.
(A lot of these say just "canonify:". This isn't a nice thing to show to a
user, but I don't believe canonify() will ever return failure due to a server
error, so users shouldn't actually see it, and it means we have a chance of
tracing it if reported.)

git-svn-id: svn://svn.tartarus.org/sgt/putty@6636 cda61777-01e9-0310-a592-d414129be87e