Split fatal and non-fatal errors, and change their presentation

[GrandyB]

Fatal errors would be those that prevent configs from being loaded, api key issues - anything to do with core config and overall sheet connection.

Non-fatal errors would include remote URLs returning 404, issues to do with individual cells. These are issues that shouldn't stop the flow of the application, nor flood the user with error popups; should be far less intrusive than that.

[GrandyB]

Has become a more important issue given the introduction of #38 which will permit some errors in order to not interrupt the update loop.

[gitagogaming]

is this 'error handling' present in the current version?

I dont recall what version I'm on anymore.. and had an error occur the other day due to a file being misnamed which caused everything to disappear and made a real ugly experience...