Decode a percent encoded (url encoded) utf8 string

Codepoint not allowed in percent encoded string are included in the output unmodified.
Broken percent encoded bytes still return an error.

* Percent encoded bytes ("%HH" where HH is the two-digit hexadecimal representation of the byte),
or sequences of those, are converted to the corresponding codepoint(s) in utf8
* The plus character "+" is converted to a space " ".