CanteraError inerits from std:exception, so now it has a what() method
that is used to print a message describing the exception. Adding an
exception to the Cantera error stack now requires explicitly calling
the .save() method.
The "cantera/kernel" directory is now part of the include path only
when building the Cantera kernel. All other modules include files in
this directory as "kernel/header.h".
This fixes a problem distinguishing between kernel/Kinetics.h and
kinetics.h on Windows.