Executor: Don't use Process.ExitCode, unless the process has exited

[radical]

Once the AfterAll signal is received, and the process doesn't exit within 2s, we try to kill the process tree with SIGTERM. But the process might not immediately quit, so trying to access process.ExitCode with the process still running throws an exception:

[2022/03/17 05:48:51][INFO] // AfterAll
[2022/03/17 05:48:53][INFO] // The benchmarking process did not quit on time, it's going to get force killed now.
[2022/03/17 05:48:53][INFO] Unhandled exception. System.InvalidOperationException: Process must exit before requested information can be determined.
[2022/03/17 05:48:53][INFO]    at System.Diagnostics.Process.EnsureState(State state)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Toolchains.Executor.Execute(Process process, BenchmarkCase benchmarkCase, SynchronousProcessOutputLoggerWithDiagnoser loggerWithDiagnoser, ILogger logger, ConsoleExitHandler consoleExitHandler, Int32 launchIndex)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Toolchains.Executor.Execute(BenchmarkCase benchmarkCase, BenchmarkId benchmarkId, ILogger logger, ArtifactsPaths artifactsPaths, String args, IDiagnoser diagnoser, IResolver resolver, Int32 launchIndex)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Toolchains.Executor.Execute(ExecuteParameters executeParameters)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkRunnerClean.RunExecute(ILogger logger, BenchmarkCase benchmarkCase, BenchmarkId benchmarkId, IToolchain toolchain, BuildResult buildResult, IResolver resolver, IDiagnoser diagnoser, Int32 launchIndex)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkRunnerClean.Execute(ILogger logger, BenchmarkCase benchmarkCase, BenchmarkId benchmarkId, IToolchain toolchain, BuildResult buildResult, IResolver resolver)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkRunnerClean.RunCore(BenchmarkCase benchmarkCase, BenchmarkId benchmarkId, ILogger logger, IResolver resolver, BuildResult buildResult)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkRunnerClean.Run(BenchmarkRunInfo benchmarkRunInfo, Dictionary`2 buildResults, IResolver resolver, ILogger logger, List`1 artifactsToCleanup, String resultsFolderPath, String logFilePath, Int32 totalBenchmarkCount, StartedClock& runsChronometer, Int32& benchmarksToRunCount)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkRunnerClean.Run(BenchmarkRunInfo[] benchmarkRunInfos)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkSwitcher.RunWithDirtyAssemblyResolveHelper(String[] args, IConfig config, Boolean askUserForInput)
[2022/03/17 05:48:53][INFO]    at BenchmarkDotNet.Running.BenchmarkSwitcher.Run(String[] args, IConfig config)
[2022/03/17 05:48:53][INFO]    at MicroBenchmarks.Program.Main(String[] args) in /home/helixbot/work/A60608D9/p/performance/src/benchmarks/micro/Program.cs:line 42

[radical]

cc @adamsitnik @lewing

[radical]

For a future PR - the exception should have been caught somewhere, and not crashed the whole thing. It could maybe be caught in BenchmarkRunnerClean:Run, or RunCore, but I wasn't sure what should be done in that case.

[adamsitnik]

Thank you for the fix @radical!