Rename `browsertest-errors` directory

`browsertest-errors` should be renamed to `screenshots` or so because it gets other screenshots too. Can consider the overall report directory structure.

Search the repository for the `browsertest-errors` directory and all references to it, then inspect the surrounding report directory structure. Determine whether the requested change is only a rename or also a broader restructuring; done means the directory name accurately covers all screenshots and every reference remains consistent.

Written by the indexing model from the issue text.