Better error for invalid multipolygon

[colings86]

Closes #7126

[clintongormley]

I actually like the message in the assertion better :) How about:

Invalid polygon: Two edges cross the date line at the same position

[colings86]

@clintongormley the problem is that the message in the assertion is not the only reason for the exception being thrown (i.e. it was always the wrong message). For example, in this case there are not two edges crossing the date line at the same position, the problem is that the polygon representing the hole is not inside the main polygon.

[clintongormley]

ahhh right, ok, how about "Invaild shape: Hole is not within polygon"

[jpountz]

LGTM