Add sandbox image build failure diagnostics

[eabatalov]

Summary

• add post-failure diagnostics for sandbox image build/import failures once the builder sandbox proxy is available
• append concise memory/disk guidance when dmesg shows OOM evidence or df reports >=95% usage
• preserve existing error structure and cleanup behavior

Tests

• cargo test -p tensorlake sandbox_images --lib
• cargo test -p tensorlake --lib
• git diff --check