Error handling for unexpected column names

[acharliekelly]

Right now, if a column in the spreadsheet is names something other than what the code expects, the app will load the header and footer, and then hang forever. Not only is there no user feedback, there's nothing in the console or even the server output. So, if you don't know that somebody just changed a column name, you might never figure out what went wrong.

Need to add some debugging output to the console, and something for the user besides the infinite progress circle. That way they know that there's a problem and the page isn't going to load, no matter how long they wait.

[acharliekelly]

Duplicates issue #284, closing