Bugfix: Grid number in error messages.

The error message refers to cgmsgrid, which is not defined in this
context (or anywhere else in the project). I changed it to refer to grid
instead.
Fixed a minor bug with the construction of an error message and replaced if by an elif. Would fail for `mode="pp"`
Merge pull request #3 from marekpetrik/develop

Bugfix: Grid number in error messages.