Fix utf8.len

[d-enk]

doc/doc/reference/reference_lua/utf8.rst

Lines 238 to 239 in 99a235a

	:return: the number of characters in the string, or between start and end
	:rtype: number

Nothing about error

doc/locale/en/reference/reference_lua/utf8.pot

Lines 318 to 320 in 99a235a

	#: ../../doc/reference/reference_lua/utf8.rst:244
	msgid "If the string contains a byte sequence which is not valid in UTF-8, each byte in the invalid byte sequence will be counted as one character."
	msgstr ""

That's not so

tarantool> utf8.len("123\xf0\xf0")
---
- null
- 4
...

Lua5.3 doc

Returns the number of UTF-8 characters in string s that start between positions i and j (both inclusive). The default for i is 1 and for j is -1. If it finds any invalid byte sequence, returns a false value plus the position of the first invalid byte.