fixed getting ivysettings.xml from walkmod-cmd as a classpath

fixed getting ivysettings.xml from walkmod-cmd as a classpath dependency.
Added also a fix to the exception handling method, this avoids nullpointer exceptions inside the catch in load method when plugin is null

Since ClassLoader.getSystemResource("ivysettings.xml") returns null when ivysettings.xml is inside a jar (walkmod-cmd), I needed to add and extra control to look for the file in the current classloader. All this changes are required to load the walkmod environment in a maven plugin.