Ask the user to configure a valid browser command if spawning it fails
rather than falling back to some arbitrary hardcoded defaults.

This avoid spawning an unexpected browser when the configured one is
wrong, and gives the user a chance to correctly fix the preference.

git-svn-id: https://geany.svn.sourceforge.net/svnroot/geany/trunk@5918 ea778897-0a13-0410-b9d1-a72fbfd435f5