Fixed #1039 : what if the tag doesn't exist?

As described in the issue added below fix :
1>added fatal_dialog (Aw Shucks) message if the user tries to load with tag which doesn't exist.
2>provided two options in dialog ignore and continue. If Ignore is pressed GUI is left with a unusable state but user can copy the error message from session info pane.
3>rejected the promise with custom error so that this can be captured in session info pane as mentioned in point 2. Reason is currently there is no error thrown we directly reload the last notebook.
4>Added this same logic for view and mini.
Screenshots attcahed after testing:
![tag1](https://cloud.githubusercontent.com/assets/391355/5408483/6eb680a8-81a9-11e4-8c49-eef15b83db1b.png)
If Ignore is clicked:
![tag2](https://cloud.githubusercontent.com/assets/391355/5408486/6ebb358a-81a9-11e4-8fe9-698d7a18a36e.png)
If continue is clicked:
![tag3](https://cloud.githubusercontent.com/assets/391355/5408484/6eb79c18-81a9-11e4-84d8-6ff066de3a87.png)
View.html
![tag4](https://cloud.githubusercontent.com/assets/391355/5408482/6eb4328a-81a9-11e4-9a45-24affad730ab.png)
mini.html
![tag5](https://cloud.githubusercontent.com/assets/391355/5408481/6eb40760-81a9-11e4-8a29-0d1d772ed413.png)
shiny.html
![tag6](https://cloud.githubusercontent.com/assets/391355/5408485/6eb96a8e-81a9-11e4-80ed-1106120885fb.png)
With shiny somehow the dialogue doesn't come. I tried to figure out but couldn't looks like its handled ina  different way. 
@gordonwoodhull :Please let me know if this is the fix you were looking for.