Catch warning thrown in Mollweide projection.

Yet another pull request to silence another warning in the test suite. 

A RuntimeWarning is thrown in the MollWeide grid tests since mpl tries to transform a point for tick calculation which is outside the allowed interval. This happens due to the code that expands the tick interval to not miss any ticks. The transform calculates np.arcsin(y/sqrt(2))
where y is larger that sqrt(2)

I also slightly improved the warnings message below to clarify that this is related to ticks only.

Also some PEP8 around the code that I changed.