travis-ci: check AsciiDoc/AsciiDoctor stderr output

travis-ci: check AsciiDoc/AsciiDoctor stderr output

`make` does not necessarily fail with an error code if
Asciidoc/AsciiDoctor encounters problems. Anything written to stderr
might be a better indicator for problems.

Ensure that nothing is written to stderr during a documentation build.

The redirects do not work in `sh`, therefore the script uses `bash`.
This shouldn't be a problem as the script is only executed on TravisCI.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>