more descriptive error message for health check not found

[danawillow]

Fixes #18.

Previous error message:

* google_compute_target_pool.is-18: Error creating TargetPool: googleapi: Error 404: The resource 'projects/graphite-test-danahoffman-tf/global/httpHealthChecks/is-18' was not found, notFound

Current error message:

* google_compute_target_pool.is-18: Health check is-18 is not a valid HTTP health check

or

* google_compute_target_pool.is-18: Health check https://www.googleapis.com/compute/v1/projects/graphite-test-danahoffman-tf/global/httpsHealthChecks/is-18 is not a valid HTTP health check

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

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