[gnome-boxes] Throw error from correct place

[Zeeshan Ali Khattak <zeeshanak src gnome org>]

commit 76c4627df4dd3055c91bd8c6a18d60aa13111558
Author: Zeeshan Ali (Khattak) <zeeshanak gnome org>
Date:   Thu Nov 24 05:42:49 2011 +0200

    Throw error from correct place
    
    https://bugzilla.gnome.org/show_bug.cgi?id=664766

 src/installer-media.vala |    2 --
 src/os-database.vala     |    8 ++++++--
 2 files changed, 6 insertions(+), 4 deletions(-)
---
diff --git a/src/installer-media.vala b/src/installer-media.vala
index 055c74e..5221485 100644
--- a/src/installer-media.vala
+++ b/src/installer-media.vala
@@ -100,8 +100,6 @@ private class Boxes.InstallerMedia : Object {
         var os_id = device.get_property ("OSINFO_INSTALLER");
         if (os_id != null) {
             os = os_db.get_os_by_id (os_id);
-            if (os == null)
-                throw new OSDatabaseError.UNKNOWN_OS_ID ("Unknown OS ID '%s'", os_id);
         }
     }
 }
diff --git a/src/os-database.vala b/src/os-database.vala
index b788d06..91b881b 100644
--- a/src/os-database.vala
+++ b/src/os-database.vala
@@ -41,8 +41,12 @@ private class Boxes.OSDatabase {
         return db.guess_os_from_media (media, out os_media);
     }
 
-    public Os? get_os_by_id (string id) {
-        return db.get_os (id);
+    public Os get_os_by_id (string id) throws OSDatabaseError {
+        var os = db.get_os (id);
+        if (os == null)
+            throw new OSDatabaseError.UNKNOWN_OS_ID ("Unknown OS ID '%s'", id);
+
+        return os;
     }
 
     public Resources get_resources_for_os (Os? os) {