Where do "internal error" error reports go?

All the errors from apport (which is the one reporting that the error occurred), will be stored in the /var/crash directory with the name of the process, PID, and other relevant information of the crash. When you send them using the tickbox, they are sent to Lauchpad (pre-release) or Ubuntu Error Tracker. This can be seen in the wiki page:

If you choose to send an error report, the alert should disappear immediately. Data should be collected (if it hasn’t been already), and reports should be sent in the background, with no progress or success/failure feedback. If you are not connected to the Internet at the time, reports should be queued. Any queued reports should be sent when you next agree to send an error report while online.