Allow overriding localizations logic, including throwing as error.

Release HURL 2.2!

Add decoupling layer for localizing HTTP errors.

This removes logic from the messages module, reduces duplication, & prepares to allow callers to provide their own localized messages.

Fix actual & potential crashes.

Integrate AppStream into normal fetch routines, version 1.3.0.0!

Improve error reporting & it's localization

Add documentation, minor exported API adjustments.

Add Accept-Language support on UNIX systems.