Decode a percent encoded (url encoded) string into a sequence of bytes

The encoded input string may only contain the following characters
"A"-"Z", "a"-"z", "0"-"9", "-", ".", "_", "~", and "+"
or the string "%HH" where H is a hexadecimal digit (case insensitive).

Any other character or broken encoding will return an error.

* The alphanumeric characters "a" through "z", "A" through "Z" and "0" through "9",
as well as the special characters "-", ".", "_", and "~" are converted to the corresponding ascii value.
* Percent encoded bytes ("%HH") are simply converted to a byte.
* The plus character "+" is converted to the ascii value of the space character " ".