Orxporter crashes when an unknown export format is passed

[shello]

When a unknown format is passed to -F, orxporter's threads crash:

x∆∆x Thread 0 failed: must be str, not type

Traceback (most recent call last):
  File "/.../orxporter/export_thread.py", line 153, in run
    final_path = dest_paths.format_path(self.path, emoji, f)
  File "/.../orxporter/dest_paths.py", line 99, in format_path
    raise ValueError('Invalid export format: ' + FilterException)
TypeError: must be str, not type

(obtaining this traceback requires PR #34)

The referenced line is:

orxporter/dest_paths.py

Line 99 in e953b4e

raise ValueError('Invalid export format: ' + FilterException)

Was this ValueError meant to be a FilterException, and the format string being concatenated to the message (instead of the FilterException class, which makes orxporter crash without a helpful message)?