Experimental: provide useful warning for invalid experimental variable values

Provide a useful warning if an invalid UUID is given in a
`CMAKE_EXPERIMENTAL_` variable.  This can occur if a different CMake
version is used than the one that the feature was implemented with, or
if there is a simple typo in the value.  It should reduce confusion with
the use of experimental features.

Co-authored-by: Brad King <brad.king@kitware.com>