[meld] meldwindow: Improve the error message for auto-merge creation failure

[Kai Willadsen <kaiw src gnome org>]

commit ed55dabab2ec36f073b887d580012ef249a42261
Author: Kai Willadsen <kai willadsen gmail com>
Date:   Fri Sep 19 06:05:51 2014 +1000

    meldwindow: Improve the error message for auto-merge creation failure

 meld/meldwindow.py |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)
---
diff --git a/meld/meldwindow.py b/meld/meldwindow.py
index d4b64ce..c2afbe4 100644
--- a/meld/meldwindow.py
+++ b/meld/meldwindow.py
@@ -652,7 +652,9 @@ class MeldWindow(gnomeglade.Component):
         return doc
 
     def append_filemerge(self, files, merge_output=None):
-        assert len(files) == 3
+        if len(files) != 3:
+            raise ValueError(
+                _("Need three files to auto-merge, got: %r") % files)
         doc = filemerge.FileMerge(len(files))
         self._append_page(doc, "text-x-generic")
         doc.set_files(files)