Suspend does not work when more users are logged in on other X sessions

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Describe the bug
Same happen with `systemctl suspend` command. It says there are more users logged in. Not sure if you use this command, but it works if you use `sudo` on this case, so maybe we could try to suspend and if it fails for this reason we could display an alert explaining this and asking for authentication.
I think this issue does not happen when using the power-off action, but I'm not sure.

### Expected behaviour

**Suspend should not fail silently**. Add an error/alert message and/or ask for authentication.

## Platform Information
- OS: elementary OS
- OS Version 5.1.7 Hera
- Hardware info Intel® Core™ i7-3632QM CPU @ 2.20GHz Toshiba L845 notebook

- [ ] I'm using the latest version from git that I've manually compiled
- [X] I'm using the latest released stable version