Feature: Expose color-eyre Handler sections

[sclu1034]

Motivation

For a UI error dialog, I'd like to be able to only displays specific HelpInfo sections and separately from the error message. I'd also like to arrange and style the header and text.

Feature suggestion

Expose the list of HelpInfos in a color_eyre::Report. Either on the Report directly, or on Handler, where they can be accessed with err.handler().downcast_ref::<Handler>().