Revert and fix "Revert "Modify -—with-extensions to throw extension dependency errors""

Revert and fix "Revert "Modify -—with-extensions to throw extension dependency errors""

This reverts commit cc7ec36a577161bba06159abd15efddc8a4f745d.

It makes the following changes from the original patch:
* findExtensions() will not consider config-extension-not-found as an
  error.
* Fixed a bug where extension info wouldn't actually be returned by
  findExtensions().
* When an extension's dependency doesn't exist, wrap the
  config-extension-not-found error in config-extension-dependency.
  This makes it more clear, and prevents the error from being ignored
  per the first bullet.
* maintenance/install.php will use ->text() rather than ->parse()
  when printing thrown errors.
* Change the stuff done to make phan happy.

Bug: T225512
Change-Id: I7d29700e8b7e91841556847d669b350cbd306fe6