Keep benchmarking errors

[gflegar]

Currently, we're dropping all the errors that happen in the benchmarks.

The run_all_benchmarks.sh script redirects all the stderr output from benchmark runners to stdout, and the stdout of run_all_benchmarks.sh is redirected to /dev/null in the CMake target for running the benchmarks (defined in benchmark/CMakeLists.txt). Instead, we should create a temporary file and redirect the output there.