Report packages loading failure with a specific exit-code

[podtserkovskiy]

The packages.Load() function can use an external package provider GOPACKAGESDRIVER for third-party build-systems like Bazel or Buck2.

Having a specific exit code when external driver fails would increase observability and will help to distinguish GOPACKAGESDRIVER's failures from other kinds of failures.

[boring-cyborg]

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[podtserkovskiy]

@ldez Is there something I can do to get this PR merged?

[ldez]

I'm unsure to understand how this "increases observability and will help to distinguish GOPACKAGESDRIVER's failures from other kinds of failures".
The error message is already "unique" 🤔

Can you provide more details?