Maybe raise a keyerror is a better way to process the locale not supported

[Raibows]

Because return the not actually converted sentence may confuse someone that is careless about the supported locales.
And I think raise an error early can stop future big error for user codes.

zhconv/zhconv/zhconv.py

Lines 235 to 254 in 078838e

	def convert(s, locale, update=None):
	"""
	Main convert function.
	:param s: must be `unicode` (Python 2) or `str` (Python 3).
	:param locale: should be one of ``('zh-hans', 'zh-hant', 'zh-cn', 'zh-sg'
	'zh-tw', 'zh-hk', 'zh-my', 'zh-mo')``.
	:param update: a dict which updates the conversion table, eg.
	``{'from1': 'to1', 'from2': 'to2'}``
	>>> print(convert('我幹什麼不干你事。', 'zh-cn'))
	我干什么不干你事。
	>>> print(convert('我幹什麼不干你事。', 'zh-cn', {'不干': '不幹'}))
	我干什么不幹你事。
	>>> print(convert('人体内存在很多微生物', 'zh-tw'))
	人體內存在很多微生物
	"""
	if locale == 'zh' or locale not in Locales:
	# "no conversion"
	return s

Let's see a mistake that may confuse the users in using the locale:

inputs = "我幹什麼不干你事"
print(convert(temp, locale='zh_cn'))
#output the same as the inputs but no warnings or erros 

Maybe many users like me use zh_cn instead of the correct zh_cn by mistake sometime.