Improve error message.

This is in response to an email on the mailing list that showed an error message that wasn't particularly helpful.
Improve documentation.
Merge pull request #31 from bangerth/improve-error-message

Wolfgang. 
This is in response to an email on the mailing list that showed an error message that wasn't particularly helpful.
Improve error message.
Merge pull request #32 from bangerth/doc-fixes

Improve documentation of the mapping cartesian class.