User messages are added to the new recovery dialog

When login is not successful, a new recovery dialog is prompted with a
message about the previous error.
New certificates are added without creating files

LeapHttpClient can fetch the main CA certificate downloaded from the
provider and add it to its in-memory keystore, without saving the
certificate in a file.

This is a very important feature, I think it can be cherry picked (it has
little modifications, and very localized).
It's very important because authentication does not work without this
bug fix in the latest branches, because I removed the code that saved
the certificate in a file but didn't test the authentication part.
ConfigurationWizard backstack and exit behavior.

Feature #3023 first solution.

I don't like to tell anything to the user when s/he wants to exit
pressing the back button. I assume s/he is saying: "Wanna go out!!"

I've implemented the behaviour written in UI Rev II.

If a user selects a new provider when coming back from Switch Provider,
the previous provider is forgotten and we assume s/he wants to change of
provider but he is not sure to which. That means that previous provider
configuration will be removed (that means there is no provider selected
and Dashboard will not be launched before a new provider is selected).