Ignore a new pep8-naming error

The new error <PyCQA/pep8-naming#157> strikes me
as a little overzealous: while most exceptions are errors, there are
some Exception subclasses that are *not* properly considered errors
(e.g., when they're only a base class for other error classes).

Author: sampsyo

setup.cfg

@@ -26,6 +26,7 @@ ignore =
     FI50,  # `__future__` import "division" present
     FI51,  # `__future__` import "absolute_import" present
     FI53,  # `__future__` import "print_function" present
+    N818,  # Exception subclasses should be named with an Error suffix
 per-file-ignores =
     ./beet:D
     ./docs/conf.py:D