librecipe/execute: fallback to polling when inotify fails

When user ran out of inotify watch, starting wrappers using
wait-for-files is not possible:

    AssertionError: Traceback (most recent call last):
      File "/srv/slapgrid/slappart15/tmp/tmpzurwmg4q/wrapper", line 13, in <module>
        sys.exit(slapos.recipe.librecipe.execute.generic_exec(['/bin/echo', 'done'], wait_list=['/srv/slapgrid/slappart15/tmp/tmpzurwmg4q/wait']))
      File "/srv/slapgrid/slappart15/srv/project/slapos-clean/slapos/recipe/librecipe/execute.py", line 100, in generic_exec
        _wait_files_creation(wait_list)
      File "/srv/slapgrid/slappart15/srv/project/slapos-clean/slapos/recipe/librecipe/execute.py", line 30, in _wait_files_creation
        watchdescriptors = {inotify.add_watch(dirname,
      File "/srv/slapgrid/slappart15/srv/project/slapos-clean/slapos/recipe/librecipe/execute.py", line 30, in <dictcomp>
        watchdescriptors = {inotify.add_watch(dirname,
      File "/srv/slapgrid/slappart15/srv/project/venv/lib/python3.9/site-packages/inotify_simple.py", line 110, in add_watch
        return _libc_call(_libc.inotify_add_watch, self.fileno(), fsencode(path), mask)
      File "/srv/slapgrid/slappart15/srv/project/venv/lib/python3.9/site-packages/inotify_simple.py", line 39, in _libc_call
        raise OSError(errno, os.strerror(errno))
    OSError: [Errno 28] No space left on device

This catches inotify errors and fallback to simple polling in that case.