Refactor the error type for hext to RGB(A)

[Ogeon]

Description

As mentioned in #347, the error type for FromStr can be improved a bit:

• Remove the From<&'static str> implementation. It's ambiguous and not necessary for the library.
• Remove the wrapped &str values. Each error has only one message anyway, or should have, so they can be part of the formatting instead.

Motivation

Focuses the purpose of the error type and removes ambiguity.