Better error handling and styles in generate srcbook modal

[benjreinhart]

• If generation fails, we show an error message. This PR makes it so that error message is dismissed the next time the user clicks "generate" or "try again"
• Add correct styles for API key warnings
• Add correct styles for errors
• Special case handling of invalid API key error (ran into this locally)
• Add "try again" link to error message. I'm a bit torn on this since I expect most cases probably won't succeed when trying again, but added it anyways.

When user never set api key

When user set api key but the key is invalid according to openai

Generic error

[nichochar]

Very nice!