Tor Browser silently fails if dbus-glib is not installed on Linux

Summary

Tor Browser relies on externally installed dbus-glib (as noted in #10014 (closed)). I understand this is wontfix, but from a user perspective, if dbus-glib is not installed, Tor Browser just silently fails to launch, which makes it hard to debug this problem. I suggest providing some kind of indicator if there's a missing dependency.

Steps to reproduce:

1. Uninstall dbus-glib: on Debian, sudo apt remove libdbus-glib-1-2, on Fedora, sudo dnf remove dbus-glib
2. Try to run Tor Browser: ./start-tor-browser.desktop

(This was tested with tor-browser-linux64-12.5.1_ALL on Debian 11 and Fedora 38, both as VMs on Qubes OS.)

What is the current bug behavior?

Output:

Launching './Browser/start-tor-browser --detach'...

But no window ever appears.

What is the expected behavior?

The user gets some indication that the browser failed to launch (so they don't just sit there waiting for it) and why (so they can fix the problem). This could just be output to the command line, but ideally it would appear as a dialog since the user might not be launching Tor Browser from the command line. It might say something like "Failed to launch Tor Browser due to missing dependency: dbus-glib" and might even tell them the command to install this dependency.