Be more precise about exactly what fails when something does.

Relying upon set -e to abort things is sort of OK (it is not
a recommended option to use in general - too many odd special cases),
but only if user can work out from the "build failed" what actually
went wrong.

Tested only on amd64 build (for this, i386 is the same) - if anyone
has problems on builds for other systems, please let me know.  However
the changes affect only failure paths, the most likely problem would
be for a build to fail to halt on an error, and I hope I have avoided
that.  There should be no difference at all to error-free builds.

(kre)