add error message for duplication of the repository id.

[tamada]

In add subcommand, the repository id was exist, the error message is repository exist.
However, the causes of the error are two patterns: already registered, and duplicated repository ids between the different repositories.
The add command should identify the causes.

[tamada]

In current implementation, we cannot add a repository to another groups by using rrh add command.
rrh add command generates error and refuses the add command.
Now, I update the routine for adding a repository to the several groups.