Shorten error messages for escalation rule count and fix edge case

What does this MR do?

Context

This MR relates to the Category:Incident Management feature set, specifically on-call schedule management and escalation policies. These allow users to route notifications regarding system alerts to the right people, by defining "Escalation Rules".

• On-call schedules are live: https://docs.gitlab.com/ee/operations/incident_management/oncall_schedules.html
• Escalation policies are close to rolling out, but are still behind the escalation_policies_mvc feature flag: &4638
• Demo projects: https://gitlab.com/gitlab-examples/ops/incident-setup/everyone/tanuki-inc/-/oncall_schedules, https://staging.gitlab.com/gitlab-org/monitor/monitor-sandbox/-/escalation_policies

Changes

This refactors the error handling for the upper limit of escalation rules per policy, moving the check from the model validations into the create/update services.

1. this brings the error message earlier in the flow, so we don't try to create all of the new escalation rules individually
   • trying to create each rule takes unneeded time, and results in an error-per-rule, instead of a single error
2. there was something funky in the logic of the validation, causing it to behave inconsistently
   • I was able to create >10 rules for a policy, as well as blocked with <10 (all during policy edits) - I'm still not entirely sure why, but moving the logic simplifies it

Screenshots or Screencasts (strongly suggested)

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Security

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