getNumberOfProcesses() shouldn't die if it fails to get system info.

On my computer, `nproc` is not available. This should not be a fatal error, because we can try other ways to check the number of cores, and fall back to only using one thread if that fails too.

However, what actually happens is that `ProcessManager.runProcess` sees that we couldn't run `nproc`, and calls `Log.error()`, which terminates the program. We need to set the `ignoreErrors` flag to true to avoid this.