Improve error descriptions for unauthorized_client (#21292)
Improve error descriptions for unauthorized_client (#21292)

Fixes #21282


As suggested by the [OAuth RFC](https://www.rfc-editor.org/rfc/rfc6749)
(quoted below), it's helpful to give more detail in the description

> error_description
OPTIONAL. Human-readable ASCII
[[USASCII](https://www.rfc-editor.org/rfc/rfc6749#ref-USASCII)] text
providing **additional information, used to assist the client developer
in understanding the error that occurred.**
Values for the "error_description" parameter MUST NOT include characters
outside the set %x20-21 / %x23-5B / %x5D-7E.
go-gitea/gitea,go-gitea/gitea,go-gitea/gitea