Loading message to user when switching sheets

[paulsjv]

As a user I want to know that the application is loading when I choose a different sheet from the dropdown at the top of the application.

Given there are multiple sheets to choose from
When I select a new sheet
Then I expect to see a loading message and the screen darken while loading

Given I have selected a sheet from the drop down and it is currently loading
When the application has loaded
Then I expect the loading message and the darken screen to disappear and be able to use the application as normal

Given I have selected a sheet from the drop down and it is currently loading
When the application has an error loading
Then I expect a error message with a dismiss button

Given there is an error message when loading the new sheet
When I dismiss the error message
Then I expect the message to disappear and the darken screen to disappear and the ability to continue the use of the application in the previous state