I’m interested in tracking errors/warning (javascript + just general logic errors) on my site so that I can diagnose issues that users are seeing before they report them. Is the best way to do this to use custom variables or is there something baked into the API for tracking client side errors?
Maybe look into this github issue (especially the last comment): https://github.com/piwik/piwik/issues/4977
It is working in general but only very basic. We simply track all errors as events for now but hope to provide similar enhanced features some day than others do
Thx! I wish that was documented somewhere 
The basic JavaScript error tracking and reporting in Matomo (browser console error messages) is now documented @ https://matomo.org/faq/how-to/how-do-i-enable-basic-javascript-error-tracking-and-reporting-in-matomo-browser-console-error-messages/