spi2davex: unspecific BadRequest error instead of error code matching the RepositoryException

the JsonDiffHandler#NodeHandler in the server part of the jcr remoting may only throw IOException
if an error occurs. this results in unspecific BadRequest responses even if the problem source was
something very specific such as e.g. a locked node.

after having a first glance at this i think that making DiffException a subclass of IOException would
allow to generate much more specific responses codes that even include the original exception
details.

i will attach a patch as i didn't had time to carefully test it. [the conformance tests passed].