Fixed bug in exporting notebook to HTML

[techwithtim]

There are some notebooks that contain special lines or cells that cause NBConvert to fail to export them. If you'd like an example of such notebook I've attached a link to one. Previously when attempting to export this notebooks no error message would be displayed. In other words, I had not handled some edge cases where the export fails in one specific method (check files to see where). This PR simply handles the case where the export fails and displays the appropriate error message to the user.

Link to download notebook you can use to Repro: https://microsoft.sharepoint.com/teams/python/Shared%20Documents/test%20.ipynb%20notebooks/Core_Learning_Algorithms.ipynb

Simply open the notebook and try to export it to .html (you will need the latest master branch changes).

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR).
• Title summarizes what is changing.
• Has a news entry file (remember to thank yourself!).
• Appropriate comments and documentation strings in the code.
• Has sufficient logging.
• Has telemetry for enhancements.
• Unit tests & system/integration tests are added/updated.
• Test plan is updated as appropriate.
• package-lock.json has been regenerated by running npm install (if dependencies have changed).
• The wiki is updated with any design decisions/details.

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