guest: Tweak error with invalid --os-variant

Try to point people in the right direction if they specify an
unknown distro name on the cli, like was reported here:

https://bugzilla.redhat.com/show_bug.cgi?id=1576473

virt-install

@@ -304,20 +304,21 @@ def convert_old_features(options):
 ##################################
 
 def set_distro_variant(options, guest, installer):
+    distro = None
     try:
         installer.check_location(guest)
 
-        distro = None
         if options.distro_variant == "auto":
             distro = installer.detect_distro(guest)
         elif options.distro_variant != "none":
             distro = options.distro_variant
 
-        if distro:
-            guest.set_os_name(distro)
     except ValueError as e:
         fail(_("Error validating install location: %s") % str(e))
 
+    if distro:
+        guest.set_os_name(distro)
+
 
 def do_test_media_detection(conn, options):
     url = options.test_media_detection

virtinst/guest.py

@@ -259,8 +259,8 @@ class Guest(XMLBuilder):
     def set_os_name(self, name):
         obj = OSDB.lookup_os(name)
         if obj is None:
-            raise ValueError(
-                _("Distro '%s' does not exist in our dictionary") % name)
+            raise ValueError(_("Unknown OS name '%s'. "
+                    "See `osinfo-query os` for valid values.") % name)
 
         logging.debug("Setting Guest osinfo %s", obj)
         self.__osinfo = obj