Don't print scary traceback when textures cannot be found

See #860

overviewer_core/textures.py

@@ -29,6 +29,11 @@ import functools
 import util
 from c_overviewer import alpha_over
 
+class TextureException(Exception):
+    "To be thrown when a texture is not found."
+    pass
+
+
 ##
 ## Textures object
 ##
@@ -213,7 +218,7 @@ 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}. Also see <http://docs.overviewer.org/en/latest/running/#installing-the-textures>".format(filename))
+        raise TextureException("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"""