Clarify formgrader port and suppress notebook output

[jhamrick]

Fixes #472

This prints a more useful error message:

$ nbgrader formgrade
[FormgradeApp | INFO] Serving MathJax from /Users/jhamrick/miniconda3/envs/nbgrader/lib/python3.5/site-packages/notebook/static/components/MathJax/MathJax.js
[FormgradeApp | ERROR] Address already in use by another process: http://localhost:5000
[FormgradeApp | ERROR] Try running nbgrader with a different port, e.g. nbgrader formgrade --port=5001

and also is more explicit about where the formgrader is running:

$ nbgrader formgrade --port=5001
[FormgradeApp | INFO] Serving MathJax from /Users/jhamrick/miniconda3/envs/nbgrader/lib/python3.5/site-packages/notebook/static/components/MathJax/MathJax.js
[FormgradeApp | INFO] Notebook server is running at http://localhost:55550/notebooks/
[FormgradeApp | INFO] The formgrader is running at http://localhost:5001/
[FormgradeApp | INFO] --> Go to http://localhost:5001/ to access the formgrader
[FormgradeApp | INFO] Use Control-C to stop this server