sqlite: provide offending filename when open fails

Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/LocalStatusSQLite.py

@@ -67,8 +67,9 @@ class LocalStatusSQLiteFolder(BaseFolder):
             self.connection = sqlite.connect(self.filename, check_same_thread=False)
         except NameError:
             # sqlite import had failed
-            raise UserWarning('SQLite backend chosen, but no sqlite python '
-                'bindings available. Please install.'), None, exc_info()[2]
+            raise UserWarning("SQLite backend chosen, but cannot connect "
+                "with available bindings to '%s'. Is the sqlite3 package "
+                "installed?."% self.filename), None, exc_info()[2]
 
         #Make sure sqlite is in multithreading SERIALIZE mode
         assert sqlite.threadsafety == 1, 'Your sqlite is not multithreading safe.'