Add LengthOrHashMismatchError back to metadata.

22b2726 claims to add all names
that were moved to the internal _payload module back to metadata.

LengthOrHashMismatchError was not added back. Now it is.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Author: Lukas Puehringer

tuf/api/metadata.py

@@ -50,6 +50,7 @@
     DelegatedRole,
     Delegations,
     Key,
+    LengthOrHashMismatchError,
     MetaFile,
     Role,
     Root,