breaks UI

lib1_2 directory was never added to the build path, so never shipped
with org.eclipse.pde.ds.annotations.

Added directory and extra errors pointing to possible configuration
problems.

Change-Id: Id5b0770af27176829d670b2e52335f1599d9fe6b
Signed-off-by: Andrey Loskutov <loskutov@gmx.de>