Abort release, if asciidoc is missing

It is hard to figure out, why no documentation is built, if asciidoc is
not installed on the build system.

Thus, abort the build and advise to either install asciidoc or add the
--without-documentation parameter.

Change-Id: I12c5c71d4f36333eabf01515344111ee9436a07c
1 file changed