Add a passthru attribute to PHPStanTask so it can print errors

[tfedor]

Describe the bug
With checkreturn argument it is possible for build to fail, but I can't seem to find an option to show errors. The actual command is run with 2>&1. Checking the code, it looks like in background ExecTask is being executed, which needs either output, error or passthru, but I can't find a way to pass it to the task.

Steps To Reproduce
Run PHPStanTask on a code that contains error.

Expected behavior
Errors should be printed to the console.

Additional context
3.0 RC6

[mrook]

Would you prefer capturing the output in a property, or just activating passthru?