fix: remove sudoer assumption in install.sh; fix bugs in remove / rename part of install_single_binary

[gkc]

- What I did

• fix: remove assumption in install.sh that the current user is a sudoer
• fix: fix buggy code I added to install_single_binary to (1) remove $binary.old if it exists (2) rename current $binary to $binary.old
• couple of small other things
  • chore: run dart format
  • fix: srv_impl.dart: throw an SshnpError rather than an Exception if the srv binary can't be found

- How to verify it

• Tested on MacOS as a normal user
• Tested on an Ubuntu VM as root
• Tested on an Ubuntu VM as a normal linux user (WITHOUT sudo permission)
• Tested on an Ubuntu VM as a normal linux user (WITH sudo permission)
• For each of the above, tested the following scenarios
  • New installation - i.e.
    • no srv or sshnpd in /usr/local/bin
    • no ~/.local/bin directory
  • Repeat installation
  • Remove ~/.local/bin and repeat installation
  • Remove /usr/local/bin/srv* and /usr/local/bin/sshnpd* and repeat installation