notify: always check context before retrying
notify: always check context before retrying

This addresses the misleading error messages reported in #212
Explanation: https://github.com/prometheus/alertmanager/issues/282#issuecomment-237784895

Fixes #282
tinytub/alertmanager,Kellel/alertmanager,tyrken/alertmanager,brancz/alertmanager,joehandzik/alertmanager,tyrken/alertmanager,tyrken/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,joehandzik/alertmanager,joehandzik/alertmanager,GoogleCloudPlatform/alertmanager,brunoqc/alertmanager,tinytub/alertmanager,tyrken/alertmanager,brunoqc/alertmanager,Kellel/alertmanager,prometheus/alertmanager,brunoqc/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,brunoqc/alertmanager,GoogleCloudPlatform/alertmanager,Aloomaio/alertmanager,brancz/alertmanager,brancz/alertmanager,prometheus/alertmanager,tinytub/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,Aloomaio/alertmanager,brancz/alertmanager