Check if the snapd socket exists before trying to run it (fixes #56)

src/linux.rs

@@ -201,6 +201,7 @@ pub fn run_flatpak(terminal: &mut Terminal) -> Option<(&'static str, bool)> {
 pub fn run_snap(sudo: &Option<PathBuf>, terminal: &mut Terminal) -> Option<(&'static str, bool)> {
     if let Some(sudo) = sudo {
         if let Some(snap) = which("snap") {
+            if PathBuf::from("/var/snapd.socket").exists() {
                 terminal.print_separator("snap");
 
                 let success = || -> Result<(), failure::Error> {
@@ -216,6 +217,7 @@ pub fn run_snap(sudo: &Option<PathBuf>, terminal: &mut Terminal) -> Option<(&'st
                 return Some(("snap", success));
             }
         }
+    }
 
     None
 }