otter.assign.utils.AssignNotebookFormatException show more informative error messages

[eroubenoff]

When using otter assign, error messages display the cell number where the error occurs, which isn't necessarily trivial to find in a larger notebook. For ex, in the following message, it isn't necessarily easy to find cell 143 without counting. Could this message display the question metadata, like, Found a begin question <<question name>> cell inside another question <<question name>>?

Thank you!

[chrispyles]

Good suggestion @eroubenoff! I've opened #399 which updates the exception message to look like this:

otter.assign.utils.AssignNotebookFormatException: Found a begin question cell inside another 
question (question q2_3, cell number 57)