Don't raise an internal when Clearsilver is not installed, but rather present an explicit error message.
Don't raise an internal when Clearsilver is not installed, but rather present an explicit error message.

Closes #7063.


git-svn-id: 0d96b0c1a6983ccc08b3732614f4d6bfcf9cbb42@6822 af82e41b-90c4-0310-8c96-b1721e28e2e2

rbaumg/trac,rbaumg/trac,rbaumg/trac,rbaumg/trac