Reduce noise from logged errors for bad user inputs

[jpd236]

A few instances of bad user input cause client+server errors to be logged, which causes noise w/ BugSnag and notifications, and in some cases results in less clear user-facing errors. IMO, these would ideally be handled more gracefully to avoid the noise.

From our test run with GPH, I noticed the following:

• Invalid URL when creating puzzle (looks like there's some sort of regex powering this in the MongoDB schema)
• Puzzle title is empty when creating/updating puzzle
• Guess is empty when adding answer