Update handling of format errors on pagerduty_schedule.start

[imjaroiswebdev]

Close #659

Tests cases extended...

$ 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 (10.47s)
=== RUN   TestAccPagerDutySchedule_Basic # 👈 This test was extended
--- PASS: TestAccPagerDutySchedule_Basic (20.38s)
=== RUN   TestAccPagerDutyScheduleWithTeams_Basic
--- PASS: TestAccPagerDutyScheduleWithTeams_Basic (15.55s)
=== RUN   TestAccPagerDutySchedule_BasicWithExternalDestroyHandling
--- PASS: TestAccPagerDutySchedule_BasicWithExternalDestroyHandling (11.44s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependant (18.24s)
=== RUN   TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents
--- PASS: TestAccPagerDutyScheduleWithTeams_EscalationPolicyDependantWithOpenIncidents (48.04s)
=== RUN   TestAccPagerDutySchedule_EscalationPolicyDependantWithOpenIncidents
--- PASS: TestAccPagerDutySchedule_EscalationPolicyDependantWithOpenIncidents (50.39s)
=== RUN   TestAccPagerDutyScheduleOverflow_Basic
--- PASS: TestAccPagerDutyScheduleOverflow_Basic (13.01s)
=== RUN   TestAccPagerDutySchedule_BasicWeek
--- PASS: TestAccPagerDutySchedule_BasicWeek (13.31s)
=== RUN   TestAccPagerDutySchedule_Multi
--- PASS: TestAccPagerDutySchedule_Multi (14.61s)
PASS
ok      github.com/terraform-providers/terraform-provider-pagerduty/pagerduty   215.920s