Edited error message for missing texture file(s)

Now links to the docs for more information.

overviewer_core/textures.py

@@ -213,7 +213,7 @@ class Textures(object):
                 if verbose: logging.info("Found %s in '%s'", filename, path)
                 return open(path, mode)
 
-        raise IOError("Could not find the file `{0}'. Try specifying the 'texturepath' option in your config file. Set it to the directory where I can find {0}.".format(filename))
+        raise IOError("Could not find the file `{0}'. Try specifying the 'texturepath' option in your config file. Set it to the directory where I can find {0}. Also see <http://docs.overviewer.org/en/latest/running/#installing-the-textures>".format(filename))
 
     def load_image(self, filename):
         """Returns an image object"""