Improve slack error handling
Improve slack error handling

Signed-off-by: Diogo Nicoleti <8009c039e4b3df8a5086d1a5ddbccdc5fb691e19@gmail.com>

GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,prometheus/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager