(GEP-199) Prevent NullPointerException when module build fails

A failing module build operation will yield a null tarball File which
in turn resulted in a NPE when attempting to publish. This commit
ensures that no publishing attempt is made when this happens and
that the error is propagated correctly.
(GEP-196) Better handling of folder exclusion patterns

Rename exclusion_patterns to folder_exclusion_patterns in the
geppetto-rc.json file to make it more clear that this is for
folders, not individual files.

Check patterns when parsing the json file so that they do
not contain '/' or '\\'