Return error from getMonitorID instead of panic

[guineveresaenger]

Description

fix: Adds error handling and slice length check for getMonitorId synthetics resource helpers.

Fixes #2631

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

Checklist:

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• My commit message follows conventional commits
• My code is formatted to Go standards
• I have performed a self-review of my own code
• 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.

How to test this change?

I am a little unclear on how to build/test this provider locally. The contributor instructions do not say how to set up my TF configuration to use a local provider. Unfortunately, internet results vary as well.

I also am unable to run make test; I get errors relating to build constraints:

 make test
tools.go:7:2: import "github.com/bflad/tfproviderlint/cmd/tfproviderlint" is a program, not an importable package
=== terraform-provider-newrelic === [ tools            ]: Installing tools required by the project...
package github.com/newrelic/terraform-provider-newrelic/v2/tools: build constraints exclude all Go files in /Users/guin/go/src/github.com/guineveresaenger/terraform-provider-newrelic/tools
make: *** [tools] Error 1

To test, I would build this provider locally and run the program that triggered the panic and hopefully get an error instead!
update: I figured out how to attach a local provider binary and with that I get an error as expected.

I am very happy to work with you on testing but I'd like some pointers on how to set up a local provider and why make is failing for me.

[CLAassistant]

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[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
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[pranav-new-relic]

Thank you for the contribution, @guineveresaenger - we would like to take this further via PR #2640 as our team believes there is scope for further error handling and better test coverage of such a scenario, so we shall continue to build via the PR linked above to resolve the issue you've posted :) thank you again for helping set a direction and giving us hints, via this PR!