Error reports from most language kernels running in IPython/Jupyter Notebooks indicate the line on which the error occurred, but (at least by default) no line numbers are shown in notebooks.
How is it possible to add the line numbers to IPython/Jupyter Notebooks?
CTRL - ML toggles line numbers in the CodeMirror area. See the QuickHelp for other keyboard shortcuts.
In more details CTRL - M (or ESC) bring you to command mode, then pressing the L keys should toggle the visibility of current cell line numbers. In more recent notebook versions Shift-L should toggle for all cells.
If you can't remember the shortcut, bring up the command palette Ctrl-Shift+P (Cmd+Shift+P on Mac), and search for "line numbers"), it should allow to toggle and show you the shortcut.