Improve feedback for duplicate tag names

• Create a tag named NXS-3525
• Make a few more commits
• Try to tag another commit (more recent or older) as NXS-3525

=> I get a "tag created" message whereas I should be told that the tag already exists
=> But the tag is not overwritten, so it's simply a matter of giving the proper feedback in this case.

Same behaviour with release tags. The only way to change the commit linked to a specific tag is to delete the tag first.