Fix stupid, broken class loader behaviour.

Why throw a ClassNotFoundException that the caller is expecting,
when you can completely break the ClassLoader contract and return
null? YAY. </snark>

Change-Id: I1d2aff71db8d0efde8d15f87e50edad962a2c52b
Reviewed-on: http://gerrit.dmdirc.com/2931
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>