Don't fail documentation build on examples exceptions

Don't fail documentation build on examples exceptions

If example code outputs an error when run,
ipython (ipython:: blocks) now fails the whole build by default,
and tools/nbgenerate.py (.ipynb files) now leaves out that file
(leaving a broken link in the examples index).

As some examples use downloaded data and are hence expected to be
unrunnable on a Debian buildd, this instead keeps them
(::ipython with error output, .ipynb without output).

Author: Rebecca N. Palmer <rebecca_palmer@zoho.com>
Forwarded: not-needed

Gbp-Pq: Name sphinx_ignore_errors.patch