builder: unwrap MultiStatus objects

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
builder: validators produce build markers, not classpath markers

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
Partially fix #801: add warning when bnd/Eclipse source/test folders disagree

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>