- prevent create reop on existed path

- fix update lable break connection with repository

- create new label

- fix isRead check
- fix paging

If users are added to gogs due a reverse proxy automatically, their password is
set to their initial username. If their username is shorter than 6 characters,
they can never set a password. But since this password is required for some
operations (e.g. repository deletion), they could not use all features of gogs
until now.

This is because git command line shows the failure reason only if Content-Type is text/plain.