Record stderr of sim during testing and print it contents in log if it fails.

Summary:
When `sim` process fail to run xctool commands xctool silently ignores all errors printed to stderr and is not logging them at all. There could be issues in future that would be hard to find without that logs.

In this commit we are saving stderr to string and print it in case when `sim` process and testing fails.

Test plan:
1) Simulated bad input argument of `sim` process.
2) Ensured that stderr contents were printed.