Cause certs:remove to return non zero on error

This was caught when writing the certs.bats. This patch will return
non zero on error instead of zero which can casue false negatives
in testing.

plugins/certs/commands

@@ -154,7 +154,7 @@ case "$1" in
       rm -rf $APP_SSL_PATH
       plugn trigger post-domains-update $APP
     else
-      dokku_log_info1 "An app-specific SSL endpoint is not defined"
+      dokku_log_fail "An app-specific SSL endpoint is not defined"
     fi
     ;;