Do not error() if Tracker3 is not found or could not be started

Rygel simply fails to start:

$ rygel
Rygel-Message: 14:33:19.983: Rygel v0.40.0 starting…
zsh: trace trap (core dumped)  rygel

Attached a GDB stack trace.

Steps to reproduce

• Build Arch Linux package rygel or AUR package rygel-git
• Run rygel, either manually or via systemctl

(Note: the former is a binary package, whereas the latter is essentially a build script, not applying any patches, so this was confirmed on basically a fresh clone: rygel-git 1:0.40.0+16+g394a0b39-1.)