* API: Improve error handling (#26934)

* New ErrUserAlreadyExists error has been introduced

* Create user endpoint returns 412 Precondition Failed on ErrUserAlreadyExists errors

* Make ErrUserAlreadyExists error message clearer

Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>

* Use errors.Is instead of equality comparator on AdminCreateUser handler

* Improve sqlstore/user test definition

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Improve sqlstore/user tests for ErrUserAlreadyExists cases

* Remove no needed string fmt and err declaration on sqlstore/user tests

* Code improvements for sqlstore/user tests

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Use err.Error() instead of sentinel error value on AdminCreateUser

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Add ErrUserAlreadyExists handling for signup & org invite use cases

Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>
Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>