chore: improve error message, since version is nil

[bschaatsbergen]

When hc-install looks for a release that we don't have binaries for (e.g. terraform 1.0.0 darwin/arm64), we print an error message containing a nil:

 $ ./hc-install install -version 1.0.0 terraform
hc-install: will install terraform@1.0.0
failed to install terraform@1.0.0: no ZIP archive found for terraform <nil> darwin/arm64

At first, I assumed there might be a bug in hc-install because of seeing that nil. After checking the Release index I noticed we started building darwin/arm64 binaries starting from 1.1.

I'm assuming that the nil is the result of marshaling the Release API response, which can't list the product and doesn't return a version?

This PR changes the error message to:

hc-install: will install terraform@1.0.0
failed to install terraform@1.0.0: darwin/arm64 ZIP archive for terraform not found

[bschaatsbergen]

Closing this in favor of #192