handle and report errors

[crazy-max]

Currently when a benchmark or test fails, the workflow just error out and doesn't report anything: https://github.com/moby/buildkit-bench/actions/runs/10938717148/job/30367598076

--- FAIL: BenchmarkBuild/BenchmarkBuildHighParallelization256x/ref=v0.11.0/run=3
    run.go:255: 
        	Error Trace:	/src/util/testutil/run.go:255
        	            				/src/util/testutil/run.go:256
        	            				/src/test/build_test.go:22
        	Error:      	Should be true
        	Test:       	BenchmarkBuild
--- FAIL: BenchmarkBuild
FAIL
exit status 1
FAIL	github.com/moby/buildkit-bench/test	1528.528s

It should instead continue, report the error and mark failure on the website.