Fix Basic_22 robot test

When adding a juju relation that already exists
a JujuAPIError exception is raised.

The "already exists" message is contained in
JujuAPIError.error_code field.

Exception is handle in an except block and
is not propagated.

Change-Id: I0ac0d875cb9580959a474e486595144473873d24
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>