Chore: Log actual error when oauth pass thru fails (#20419)

Logs the actual error when failing to retreive access token
when OAuth pass true is enabled for a datasource.

Ref: #20407
(cherry picked from commit dfa2f3d2fb)

pkg/api/pluginproxy/ds_proxy.go

@@ -322,7 +322,7 @@ func addOAuthPassThruAuth(c *m.ReqContext, req *http.Request) {
 		TokenType:    authInfoQuery.Result.OAuthTokenType,
 	}).Token()
 	if err != nil {
-		logger.Error("Failed to retrieve access token from oauth provider", "provider", authInfoQuery.Result.AuthModule)
+		logger.Error("Failed to retrieve access token from oauth provider", "provider", authInfoQuery.Result.AuthModule, "error", err)
 		return
 	}