JS refactor
Error message is only relevant in specific circumstance, other circumstances are likely more serious errors
Removed docker delay with spinning
Refactored JS code to wait on page load

See linked SO question in source for information on the type of JS delay loop
we're doing.
Finished work on spinning

 - Removed a statically set port during testing
 - Made variables local so we can be sure they aren't being mis-used
 - Removed sleep
 - New parameters needed in notebook load call
 - Added main div to use for images/etc. Probably not necessary.
Fixes #15
Center the spinner
Merge pull request #18 from bgruening/loading

JS Refactor + Spinning