Stop retrying on Schedule deletion when open incidents are untraceable

[imjaroiswebdev]

When an Escalation Policy using and Schedule receives a new Incident, a snapshot of the EP gets created to trace the relation of the Incident with that EP and its dependencies during the Incident life cycle, however when a that Schedule gets moved from the EP before closing or reassigning the Incident, the traceability of the Schedule with the Incident is lost, therefore retrying on error "[Schedule can't be deleted if it's being used by an escalation policy snapshot with open incidents]" becomes meaningless, so this update is meant to return immediately after matching this scenario.

Test results

$ PAGERDUTY_ACC_SCHEDULE_USED_BY_EP_W_1_LAYER=1 make testacc TESTARGS="-run TestAccPagerDutySchedule"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run TestAccPagerDutySchedule -timeout 120m
?       github.com/terraform-providers/terraform-provider-pagerduty     [no test files]
=== RUN   TestAccPagerDutySchedule_import
--- PASS: TestAccPagerDutySchedule_import (13.91s)
=== RUN   TestAccPagerDutySchedule_Basic
--- PASS: TestAccPagerDutySchedule_Basic (22.27s)
=== RUN   TestAccPagerDutyScheduleWithTeams_Basic
--- PASS: TestAccPagerDutyScheduleWithTeams_Basic (16.23s)
=== RUN   TestAccPagerDutySchedule_BasicWithExternalDestroyHandling
--- PASS: TestAccPagerDutySchedule_BasicWithExternalDestroyHandling (13.00s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant (19.05s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOneLayer
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOneLayer (47.27s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents (54.16s)
=== RUN   TestAccPagerDutySchedule_EscalationPolicyDependantWithOpenIncidents
--- PASS: TestAccPagerDutySchedule_EscalationPolicyDependantWithOpenIncidents (60.47s)
=== RUN   TestAccPagerDutyScheduleOverflow_Basic
--- PASS: TestAccPagerDutyScheduleOverflow_Basic (14.31s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (14.31s)
=== RUN   TestAccPagerDutySchedule_Multi
--- PASS: TestAccPagerDutySchedule_Multi (16.58s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   292.056s