gcp-metadata: Internal error log is printed - "MetadataLookupWarning: received unexpected error = All promises were rejected"

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### Link to the code that reproduces this issue. A link to a **public** Github Repository or gist with a minimal reproduction.

https://gist.github.com/yoshigev/c3ab7122f306757744b1e0e919ae65d0

### A step-by-step description of how to reproduce the issue, based on the linked reproduction.

Just run `isAvailable()` method as shown in the gist, in **non GCP** environment.

The console output will show `MetadataLookupWarning: received unexpected error = All promises were rejected` warning.

### A clear and concise description of what the bug is, and what you expected to happen.

The bug was probably introduced by https://github.com/googleapis/google-cloud-node-core/commit/dd0012aec2c011a066b5f6cfe9bc1be95c5650b1.

The new code changed the thrown error of `fastFailMetadataRequest` - as explained in the comment there, the new error is `AggregateError`. The problem is, that this error is not masked in `isAvailable` as the other known errors.

cc @Dhoni77

### A clear and concise description WHY you expect this behavior, i.e., was it a recent change, there is documentation that points to this behavior, etc. **

There is no reason for this error to appear, as this is not a real problem.