Implement a fallback installation for script repository

There was the suggestion to try to install first the git transport and
them the http transport. This changeset implement this.

re #6175
Solve the problem of corrupted repository

If the user (could not wait for the installation) and canceled
the installation process, the folder for the scriptRepository may
be created, but the references and indexes are not valid. Further
attempt to use this repository may cause a segmentation fault.

Checking if the repository is empty help to prevent such case.

re #6175
Remove the double attempt to update the repository.

The update is done by the ScriptRepositoryView.

re #6175
Improve the communication with the user

re #6175
Provide better descriptive text for the users

Change only strings for giving the user a better description of the
Script Repository Installation process.

re #6175