[valadoc] libvaladoc: Improve error message on failing to load a driver

[Florian Brosch <flobrosch src gnome org>]

commit b055a1dfe3a8918802c1e8da76ae12137502e340
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Wed May 13 18:35:00 2015 +0200

    libvaladoc: Improve error message on failing to load a driver
    
    https://bugzilla.gnome.org/show_bug.cgi?id=711559

 src/libvaladoc/moduleloader.vala |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/src/libvaladoc/moduleloader.vala b/src/libvaladoc/moduleloader.vala
index 06e6fe1..b7bcae6 100644
--- a/src/libvaladoc/moduleloader.vala
+++ b/src/libvaladoc/moduleloader.vala
@@ -174,7 +174,10 @@ public class Valadoc.ModuleLoader : Object {
                }
 
 
-               reporter.simple_error (null, "No suitable driver found.");
+               reporter.simple_error (null, "No suitable driver found for libvala version " +
+                                                          "%" + int64.FORMAT_MODIFIER + "d.%" + 
int64.FORMAT_MODIFIER + "d.%" + int64.FORMAT_MODIFIER + "d. " +
+                                      "Ensure the selected vala version was installed while building valadoc 
or use --driver to select another one.",
+                                                          segment_a, segment_b, segment_c);
                return null;
        }