Differentiate between resolve_buck failing due to bad buck version and failing from unexpected error (#2696)

Summary:
A missing dependency (or other unexpected error) in resolve_buck.py can cause the CMake build to fail. It needs to gracefully fall back to the previous behavior in this case.

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Pull Request resolved: https://github.com/pytorch/executorch/pull/2696

Test Plan:
Uninstalled zstd via pip uninstall zstd.
Ran cmake ... Confirmed output message contained warning on script failure and gracefully fell back to system buck2.
Repeated above with cmake -DBUCK2=buck2-et2. Confirmed script failure warning and successful build.
Reinstalled zstd via pip install zstd.

Reviewed By: lucylq

Differential Revision: D55391962

Pulled By: GregoryComer

fbshipit-source-id: 3711546fb222169a7da954a773e9c057303778ca