py_blake2*_new_impl produces inconsistent error messages, and raises OverflowError where ValueError might be better

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-03-17.12:08:00.055>
labels = ['type-bug', '3.7']
title = 'py_blake2*_new_impl produces inconsistent error messages, and raises OverflowError where ValueError might be better'
updated_at = <Date 2017-03-17.13:17:24.223>
user = 'https://github.com/orenmn'

bugs.python.org fields:

activity = <Date 2017-03-17.13:17:24.223>
actor = 'christian.heimes'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2017-03-17.12:08:00.055>
creator = 'Oren Milman'
dependencies = []
files = []
hgrepos = []
issue_num = 29835
keywords = []
message_count = 2.0
messages = ['289757', '289759']
nosy_count = 4.0
nosy_names = ['christian.heimes', 'serhiy.storchaka', 'xiang.zhang', 'Oren Milman']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29835'
versions = ['Python 3.7']