indexer: set an error message on duplicate objects in pack
indexer: set an error message on duplicate objects in pack

While this is not even close to a fix, we can at least set an error
message so we know which error we are facing. Up to know we just
returned an error without a message.

jflesch/libgit2-mariadb,jflesch/libgit2-mariadb,jflesch/libgit2-mariadb,jflesch/libgit2-mariadb,jflesch/libgit2-mariadb,jflesch/libgit2-mariadb