cmd/mountlib: better snap mount error message

Mounting will always fail when rclone is installed from the snap package manager.
But the error message generated when trying to mount from a snap install was not
very good. Improve the error message.

Fixes rclone#8208

cmd/mountlib/mount.go

@@ -373,6 +373,9 @@ func (m *MountPoint) Mount() (mountDaemon *os.Process, err error) {
 
 	m.ErrChan, m.UnmountFn, err = m.MountFn(m.VFS, m.MountPoint, &m.MountOpt)
 	if err != nil {
+		if len(os.Args) > 0 && strings.HasPrefix(os.Args[0], "/snap/") {
+			return nil, fmt.Errorf("mounting is not supported when running from snap")
+		}
 		return nil, fmt.Errorf("failed to mount FUSE fs: %w", err)
 	}
 	m.MountedOn = time.Now()