Analyze error message types/incidence to plan content review and documentation

1. Identify all types of error text in the code, with examples. Can we categorize them?
   1. What are the various ways these are stored and displayed? Are there some UI-only errors vs. others which are backend, regarding the GitLab instance? While others are backend yet also surfaced in the UI?
2. Do we track the number of incidents of each error message for all error messages? For some?
   1. If so, which types / how can we identify which are tracked? (Do we only have numbers for GitLab.com?)
   2. Which kinds of errors are picked up by GitLab's error tracking feature? https://docs.gitlab.com/ee/user/project/operations/error_tracking.html

This information gathering is intended as a precursor to planning reviews and documentation of error text.