Fall back if there's no ContextClassLoader.

ContextClassLoader may be null if the system class loader should
be used. For some reason recently this seems to be null
intermittently when trying to load an error icon.

Fall back to the IconManager's class loader in this case.

Change-Id: I7522de991b629c5d1f490f060bd544a5f39e622b
Reviewed-on: http://gerrit.dmdirc.com/3921
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager
"@Inject"ed ctors aren't unused.

Change-Id: I7ff0b96cb8134f1bb20f962bc114ce36d571e3a1
Reviewed-on: http://gerrit.dmdirc.com/3922
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager