Nicer error messages

[dahlia]

Error messages the compiler currently prints could be nicer:

• Colorize code that has an error (when it isatty)
• Suggest possible solutions
• Link to a relative docs
• Structured error codes

Any more ideas?

We could learn many things about this topic from the Elm compiler.

[kanghyojun]

I think it is nice to see an informative error message when the required field is missing on package.toml.

$ nirum -t docs -o ./out/docs/ .
Metadata error: FieldError "targets.docs.title"