Check that proot is installed.

def check_isolation_executable_installed(isolation_executable):
    '''
    Check that proot is installed.
    '''
    cmd = [isolation_executable, '--version']
    try:
        proc = subprocess.Popen(cmd, stdout=subprocess.PIPE,
                                stderr=subprocess.PIPE)
        proc.communicate()
        return bool(proc.returncode == 0)
    except (OSError, ValueError) as e:
        if isinstance(e, ValueError) or getattr(e, 'errno', 1) != 2:  # ENOENT, no such file or directory
            raise RuntimeError('bwrap unavailable for unexpected reason.')
        return False