tag: Display error message when creating tag with invalid name

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>
Tag: Fix error message display on tag creation

The error message was displayed using information dialog. As a result,
it is displaying git-cola icon instead of error icon.

This change use the proper error dialog instead of information dialog.

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: Minarto Margoliono <lie.r.min.g@gmail.com>
Tag: Move error handling code to widget

The error message was handled in cmds.py while the success code was
handled in createtag.py. This will make it harder to maintain later.

This change move the error handling code to the same location as success
handling code.

Reported-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: Minarto Margoliono <lie.r.min.g@gmail.com>
Merge pull request #296 from mmargoliono/286

tag: Display error message when creating tag with invalid name

Closes #286

Signed-off-by: David Aguilar <davvid@gmail.com>