wrap user-defined function exceptions to error messages

While library internal code should be kept exception free, users (often using other C/C++ libraries) are free to throw exceptions to indicate rare errors. These exceptions are internally wrapped and passed up the chain to the graph->scheduler. In case the scheduler 'msgOut' is not connected these error messages are in turn converted to exceptions again.

Signed-off-by: Ralph J. Steinhagen <[email protected]>