Make ECM error message more helpful

Summary:
This error points to a nearly empty page that forces the user to navigate around
and attempt to discover the root cause of the error. The actual package name that needs
installing isn't actually listed anywhere in those docs as far as I could find.

This patch points the link to main page for more information about ECM and additionally
provides a hint for those that are just missing the proper package.

Test Plan:
Try running one of the sanitizer builds on a fresh machine.
This error will appear.
sudo apt-get install extra-cmake-modules
Build should progress once run again.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4881