Expand error message for FileTypeError (#2599)

...when Mutagen doesn't recognize the type at all.

Author: sampsyo

beets/mediafile.py

@@ -96,7 +96,7 @@ class FileTypeError(UnreadableFileError):
     """
     def __init__(self, path, mutagen_type=None):
         if mutagen_type is None:
-            msg = repr(path)
+            msg = u'{0!r}: not in a recognized format'.format(path)
         else:
             msg = u'{0}: of mutagen type {1}'.format(repr(path), mutagen_type)
         Exception.__init__(self, msg)

docs/changelog.rst

@@ -35,6 +35,8 @@ Fixes:
   non-ASCII filenames. Thanks to :user:`robot3498712`. :bug:`2592` :bug:`2593`
 * :doc:`/plugins/metasync`: Fix a crash in the Amarok backend when filenames
   contain quotes. Thanks to :user:`aranc23`. :bug:`2595` :bug:`2596`
+* More informative error messages are displayed when the file format is not
+  recognized. :bug:`2599`
 
 
 1.4.4 (June 10, 2017)