Give a useful error when no JVMCI version info can be retrieved

[smarr]

Hi:

I just ran into an issue when trying to build this repo on MacOS (since there don't seem to be binaries at the moment).

Problem: I cloned with git clone --depth 1, and got a index out of bounds on the JVMCI versions.

The following patch gives a hint how to resolve it, though, it's probably not the best way. And I don't know how mx usually communicates such errors.

Either suggestions welcome, or if you prefer, treat this simply as a "feature request".

Thanks!

/cc @dougxc

[dougxc]

since there don't seem to be binaries at the moment

The moment will be here for quite a while as stated in the release notes here.