encapsulate notification into try/catch

[mothsART]

I tried to launch gSpeech in raspbian but without xfce4-notifyd package, gSeech was blocked on:

Traceback (most recent call last):
  File "./gSpeech.py", line 356, in onExecute
    pynotify.Notification(APPNAME, _(u"No text selected."), self.icon).show()
glib.GError: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.Notifications was not provided by any .service files

I think, notification must not been bloking anyway.

[lusum]

Ok i will commit, it is only a safety check, so it should be no problems.