Currently, cells are named as 'Cell %d' % cell_num. This makes it hard to trace errors in case you have hundreds of cells.

Could the cells be possibly named? Either via cell metadata (given it's already supported anyway), or via inline comments, like # nbval: name = 'cell_name', or just # nbval: cell_name.

To enforce uniqueness, cell number can still be kept, so the output could look e.g. like this:

nbval_test.ipynb::Cell 0 [do_stuff] PASSED
nbval_test.ipynb::Cell 1 PASSED
nbval_test.ipynb::Cell 2 PASSED
nbval_test.ipynb::Cell 3 [other_stuff] FAILED
nbval_test.ipynb::Cell 4 PASSED
nbval_test.ipynb::Cell 5 [custom_name] PASSED
nbval_test.ipynb::Cell 6 PASSED