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Introduce ConnectionFailedError type.

Delay the CONFERENCE_FAILED event until the user cancels the login
dialog using the 'recoverable' event flag.

The preceding commit "fix(auth.native): trigger conference left on
cancel" did not correctly clean the redux state up on CONFERENCE_LEFT.

Triggering the 'conference left' action when the wait for owner dialog
is dismissed will let the Call Kit implementation end the call, after
a recoverable conference failed event was emitted.

Also fixes conference state reduction when 'conference left' is emitted
by room lock or auth features where the conference has not been joined
yet.

* Javadoc introduced @code as a replacement of <code> and <tt> which is
  better aligned with other javadoc tags such as @link. Use it in the
  Java source code. If we switch to Kotlin, then we'll definitely use
  Markdown.

* There are more uses of @code in the JavaScript source code than <tt>
  so use @code for the sake of consistency. Eventually, I'd rather we
  switch to Markdown because it's easier on my eyes.

* Xcode is plain confused by @code and @link. The Internet says that
  Xcode supports the backquote character to denote the beginning and end
  of a string of characters which should be formatted for display as
  code but it doesn't work for me. <tt> is not rendered at all. So use
  the backquote which is rendered itself. Hopefully, if we switch to
  Markdown, then it'll be common between JavaScript and Objective-C
  source code.

Gradually, we exploded the error of CONNECTION_FAILED in multiple
redux state properties. The explosion makes maintenance harder because
the properties have to be updated in sync. Collect them in an object
resembling an Error instance.