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This CL fixes a bug where closing the error screen sends network screen
exit message to network screen handler. Luckly this does not cause any
user visible bugs, but this is not the expected behavior.

Now closing the error screen just hides the screen. The default hide
callback will make the parent screen visible in the JS side without
modifying WizardController's current_screen_.

This CL also adds tests that verify the bug fix and that the basic flow
of connecting to a network in the error screen works.

Bug: 959340

Change-Id: I760b92bd1e1cdbe96059c7941a854f0664f93298
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1606572
Commit-Queue: Tony De Luna <tonydeluna@chromium.org>
Reviewed-by: Toni Baržić <tbarzic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659093}