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.
Filter warnings with more selective np.errstate context
Merge pull request #3282 from jenshnielsen/mollweide_warning

Catch warning thrown in Mollweide projection.