tag: Display error message when creating tag with invalid name (fix issue: #286)

When an invalid name is supplied during tag creation, there was no error
displayed to the user. It would always say "Created a new tag named [tag
name]" even though no tag is actually created.

This change will display error message instead of tag creation message
when git tag command fails.

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: Minarto Margoliono <lie.r.min.g@gmail.com>