Catch warning thrown in Mollweide projection.

This happens when plotting tries to expand
the interval for ticks beyond the defined region and do np.arcsin(y/sqrt(2))
where y is larger that sqrt(2) since the original limites matches the allowed
interval.

And Improve warnings message to clarify that this is related to ticks only.

Also some PEP8 around the code that I changed.
Merge branch 'numpy_clean_warnings' into allwarns
Merge branch 'rcwarnings' into allwarns
Merge branch 'mollweide_warning' into allwarns
Catch warnings in tightlayout test.

These are warnings that are raised because tightlayout does not work
fully automatic on gridspec. However, the point of the test is doing
this with some manual help so this works as expected.
Merge remote-tracking branch 'jenshnielsen/tight_layout_warning' into allwarns