document : do not redefined error class in mixin

Redefining class prevent the try/except from working as
it is not the same class which is raised.
Fix import order to prevent looping

product/ERP5/Document/Document.py

@@ -48,7 +48,6 @@ from Products.ERP5.mixin.cached_convertable import CachedConvertableMixin
 from Products.ERP5.mixin.text_convertable import TextConvertableMixin
 from Products.ERP5.mixin.downloadable import DownloadableMixin
 from Products.ERP5.mixin.document import DocumentMixin
-from Products.ERP5.mixin.extensible_traversable import DocumentExtensibleTraversableMixin
 from Products.ERP5.mixin.crawlable import CrawlableMixin
 from Products.ERP5.mixin.discoverable import DiscoverableMixin
 from Products.ERP5.mixin.url import UrlMixin
@@ -80,6 +79,8 @@ class DocumentProxyError(Exception):pass
 class NotConvertedError(Exception):pass
 allow_class(NotConvertedError)
 
+from Products.ERP5.mixin.extensible_traversable import DocumentExtensibleTraversableMixin
+
 class Document(DocumentExtensibleTraversableMixin, XMLObject, UrlMixin,
                CachedConvertableMixin, CrawlableMixin, TextConvertableMixin,
                DownloadableMixin, DocumentMixin, DiscoverableMixin):

product/ERP5/mixin/extensible_traversable.py

@@ -42,13 +42,11 @@ from warnings import warn
 import sys
 from base64 import decodestring
 from Products.ERP5Type.UnrestrictedMethod import unrestricted_apply
-
+from Products.ERP5.Document.Document import ConversionError, NotConvertedError
 
 # XXX: these duplicate ones in ERP5.Document
 _MARKER = []
 EMBEDDED_FORMAT = '_embedded'
-class ConversionError(Exception):pass
-class NotConvertedError(Exception):pass
 
 class BaseExtensibleTraversableMixin(ExtensibleTraversableMixIn):
   """