Design a proper error slide for the response pane

[danirod]

In case of error, currently Cartero shows a little toast at the bottom of the screen which just prints the debug message of the underlying Error.

A slide to display error messages should be created and added to the response pane stack. It should be similar to the one that is visible when a request is firstly created or opened but not sent yet:

This slide should be made visible when there is an error and should have the following components:

• A proper error icon (maybe an X?)
• The error message, and I mean the REAL error Message.
  • Stop with anyhow
    • Error messages are not translated using gettext, which is a bug.
    • It promotes a culture of not taking responsibility of the error.
  • If the error pane receives a CarteroError that is actually one of a subkind, it should always unwrap the real error message.
    • Instead of HTTP request error => extract the inner request error and use it.
    • If the error comes from glib, open it and get the error message using the glib API.