WebUI does not display Oauth2 login error

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initially https://github.com/woodpecker-ci/woodpecker/blob/42a115e19e9369d2d8bc5aaed467b6bc6759a70d/server/api/login.go#L71 https://github.com/woodpecker-ci/woodpecker/blob/42a115e19e9369d2d8bc5aaed467b6bc6759a70d/server/api/login.go#L81 ... was added to redirect to error handling.

but since our webui rewrite this somehow got missed.

currently we do: https://github.com/woodpecker-ci/woodpecker/blob/65d88be52397dbfbb63306f93d95fc4c81828608/server/api/login.go#L52-L56

but the WebUI has no way of handling that :/