Added some better error reporting and path handling when creating template paths.

We now raise UnicodeDecodeError for non-UTF-8 bytestrings (thanks to Daniel
Pope for diagnosing this was being swallowed by ValueError) and allow UTF-8
bytestrings as template directories.

Refs #8965.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9161 bcc190cf-cafb-0310-a4f2-bffc1f526a37