Cleaning up core if/else/error instances

[Rik]

12/22/15

Andreas et al.,

I made a new page and topic on the Octave Wiki
(http://wiki.octave.org/Invert_if/else/error) around further cleaning up
instances of error() in the core.

Each instance should take just a minute or two, but there are 105, so help
would be appreciated.  I added this to the list of Short Projects on the
Wiki too since this is an easy introduction for new programmers.

Cheers,
Rik