Improve error message when template is not found

The previous error message did not include the path that was
searched in. By including the path the debugging of the config
becomes way easiert (e.g. in my case I had added the '/templates'
folder which was added again by sigal).

sigal/writer.py

@@ -87,8 +87,8 @@ class AbstractWriter(object):
         try:
             self.template = env.get_template(self.template_file)
         except TemplateNotFound:
-            self.logger.error('The template %s was not found.',
-                              self.template_file)
+            self.logger.error('The template %s was not found in template folder %s.',
+                              self.template_file, theme_relpath)
             sys.exit(1)
 
         # Copy the theme files in the output dir