Fix the error message when the token is incorrect (#25701)

we refactored `userIDFromToken` for the token parsing part into a new
function `parseToken`. `parseToken` returns the string `token` from
request, and a boolean `ok` representing whether the token exists or
not. So we can distinguish between token non-existence and token
inconsistency in the `verfity` function, thus solving the problem of no
proper error message when the token is inconsistent.
close #24439  
related #22119

---------

Co-authored-by: Jason Song <i@wolfogre.com>
Co-authored-by: Giteabot <teabot@gitea.io>

services/auth/group.go

@@ -49,12 +49,22 @@ func (b *Group) Name() string {
 // Verify extracts and validates
 func (b *Group) Verify(req *http.Request, w http.ResponseWriter, store DataStore, sess SessionStore) (*user_model.User, error) {
 	// Try to sign in with each of the enabled plugins
+	var retErr error
 	for _, ssoMethod := range b.methods {
 		user, err := ssoMethod.Verify(req, w, store, sess)
 		if err != nil {
-			return nil, err
+			if retErr == nil {
+				retErr = err
+			}
+			// Try other methods if this one failed.
+			// Some methods may share the same protocol to detect if they are matched.
+			// For example, OAuth2 and conan.Auth both read token from "Authorization: Bearer <token>" header,
+			// If OAuth2 returns error, we should give conan.Auth a chance to try.
+			continue
 		}
 
+		// If any method returns a user, we can stop trying.
+		// Return the user and ignore any error returned by previous methods.
 		if user != nil {
 			if store.GetData()["AuthedMethod"] == nil {
 				if named, ok := ssoMethod.(Named); ok {
@@ -65,5 +75,6 @@ func (b *Group) Verify(req *http.Request, w http.ResponseWriter, store DataStore
 		}
 	}
 
-	return nil, nil
+	// If no method returns a user, return the error returned by the first method.
+	return nil, retErr
 }