tag-cmd: Some fixups

tag-cmd: Some fixups

* Make some error messages nicer in telling the tag name that was used.

* Move tag listing code in git.py and use this code in tag-cmd and vfs.py

* Make tag-cmd output the list of tags on stdout instead of stderr

* Don't error out when more than 2 arguments are given. When there are less
  than 2, be nice and show the usage.

* In tag-cmd, catch GitErrors that come out of git.rev_parse()

Signed-off-by: Gabriel Filion <lelutin@gmail.com>