sstate: Change overlapping files warning to a fatal error

When files overlap in the sysroot, something bad usually happened. We've had
two independent cases recently where a couple of months after one of these
warnings was shown, builds failed due to corruption.

This change moves the warning to become a fatal error. The complaint I've had
about this is that we need to tell the user what happened and more importantly
how to recover from it. If we could recover from it, great but the trouble is
we simply don't know what happened.

As a compromise, we can document several of the possible scenarios in the error
message. We don't normally go to this level of detail however in this case, I'm
lacking other viable alternatives.

I do believe it is important to stop as corruption occurs rather than letting the
build contunue into territory that is not deterministic amongst other things.

The complex message is followed by a simpler one in case the long message is too
much for the user.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>