shellcheck* targets were always reporting exit status 0.
With this fix, if shellcheck fails, then build-helper script
exits with 1.

Maintainers should use build-helper script.
End users should use Makefile, which contains targets
that can be run on pure source (without git repository).

Speeches should be generated on demand when needed
and the cdist website should be updated.

By default, was getting the following error:
curl: (92) HTTP/2 stream 0 was not closed cleanly: PROTOCOL_ERROR (err 1)

If it fails then the whole release should not fail,
signing git tag can be tried again.

Note that this fix only matters for maintainers using build-helper
script. Source archive is generated during release process and
uploaded to github, along its signature.
For 4.10.11 those files were fixed and uploaded manually after
build-helper script process has been finished. In future releases
this process will be automatic - no need for manual step that was
necessary for 4.10.11.

Resolves #751