Address: Schedules can't be deleted when they have open incidents

[imjaroiswebdev]

Address: #567

This update doesn't provide an strict solution to the issue, because unfortunately the Provider shouldn't be closing incidents on its own in an arbitrary fashion. However what it does provide is an error message containing a list with the urls pointing to each incident which needs to be resolved in order of retrying the Schedule destruction after the incidents are been resolved.

Example of error message expected...

Error: Before Removing Schedule "PE1DBNP" You must first resolve the following incidents related with Escalation Policies using this Schedule...
        https://subdomain.pagerduty.com/incidents/Q39V9R9QR34GZ5
        https://subdomain.pagerduty.com/incidents/Q37V9R8lR34GBQ
        .
        .
        .

Test cases introduced...

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 (11.89s)
=== RUN   TestAccPagerDutySchedule_Basic
--- PASS: TestAccPagerDutySchedule_Basic (14.98s)
=== RUN   TestAccPagerDutyScheduleWithTeams_Basic
--- PASS: TestAccPagerDutyScheduleWithTeams_Basic (15.12s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant (19.79s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents (28.16s) # 👈 New test case
=== RUN   TestAccPagerDutyScheduleOverflow_Basic
--- PASS: TestAccPagerDutyScheduleOverflow_Basic (13.97s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (13.21s)
=== RUN   TestAccPagerDutySchedule_Multi
--- PASS: TestAccPagerDutySchedule_Multi (13.58s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   131.184s

Depends on: heimweh/go-pagerduty#109

[stmcallister]

👍 🌮 🎉 Looks good! (assuming the conflicts are resolved 😉 )