_sandboxbuildboxrun.py: Check for buildbox-run initialization errors
If buildbox-run --capabilities
reports an error, we should consider the sandbox to be unavailable, forwarding the error message from buildbox-run.
This may happen, e.g., if buildbox-run-bubblewrap
is installed but bwrap
is not available.
See also BuildGrid/buildbox/buildbox-run-bubblewrap!11 (merged)