HPCC-7946 CATCH(expr,msg) won't compile

Rearrange jlib headers so that IException is not an opaque type.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-7946 CATCH(expr,msg) won't compile

Add example

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #4119 from richardkchapman/catch-expr

HPCC-7946 CATCH(expr,msg) won't compile

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>