diff --git a/mercurial/error.py b/mercurial/error.py
--- a/mercurial/error.py
+++ b/mercurial/error.py
@@ -138,6 +138,14 @@ class UnsupportedMergeRecords(Abort):
             hint=_('see https://mercurial-scm.org/wiki/MergeStateRecords for '
                    'more information'))
 
+class UnknownVersion(Abort):
+    """generic exception for aborting from an encounter with an unknown version
+    """
+
+    def __init__(self, msg, hint=None, version=None):
+        self.version = version
+        super(UnknownVersion, self).__init__(msg, hint=hint)
+
 class LockError(IOError):
     def __init__(self, errno, strerror, filename, desc):
         IOError.__init__(self, errno, strerror, filename)
diff --git a/mercurial/obsolete.py b/mercurial/obsolete.py
--- a/mercurial/obsolete.py
+++ b/mercurial/obsolete.py
@@ -446,8 +446,8 @@ def _readmarkers(data):
     diskversion = _unpack('>B', data[off:off + 1])[0]
     off += 1
     if diskversion not in formats:
-        raise error.Abort(_('parsing obsolete marker: unknown version %r')
-                         % diskversion)
+        msg = _('parsing obsolete marker: unknown version %r') % diskversion
+        raise error.UnknownVersion(msg, version=diskversion)
     return diskversion, formats[diskversion][0](data, off)
 
 def encodemarkers(markers, addheader=False, version=_fm0version):