fix(synthetics): add error handling to multilocation alert condition for invalid GUIDs

[abasha1234]

Description

This PR includes changes for error handling when an invalid monitor ID is provided for the resource newrelic_synthetics_multilocation_alert_condition.

Ticket : https://new-relic.atlassian.net/browse/NR-257490

Fixes # (issue) Issue: https://github.com/newrelic/terraform-provider-newrelic/issues/2631

Before the changes, getMonitorID() fails with panic on wrong monitor ID

After the changes, getMonitorID() throwing an error

Checklist:

Please delete options that are not relevant.

• [ ] My commit message follows conventional commits
• [ ] My code is formatted to Go standards
• [x] I have performed a self-review of my own code
• [ ] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes. Go here for instructions on running tests locally.

[abasha1234]

The PR change has been validated with the following cases

invalid monitor_id

monitor_id as empty string

[pranav-new-relic]

Awesome 👍