Improve error formatting for CLI applications

[brendanzab]

Improve error formatting for CLI applications

[Marwes]

For LSP there are some things to keep in mind.

• The first line of the message should have enough information that it and the line where the error occurred is enough to fix the problem. This is because editors tend to require additional action to view the rest of the message.

• Colors can't be used to mark important sections of the message. Instead marked locations must be done via ^^^^~~~ etc.

• On the other hand the editor will mark at least the line at which the error occured so drawing out spans in the message might not be as important, unless they involve multiple spans (currently the LSP only supports one span per diagnostic but RLS is experimenting with a way to group multiple diagnostics into one rust-lang/rls#681).

[brendanzab]

At the moment I'm thinking we'd want to have common Diagnostic struct that tries to stick to the 'semantic model' of a diagnostic, leaving ultimate 'styling' to downstream consumers, like the CLI formatter or LSP. We'd simplify/throw away some info when supporting the LSP for example.

[brendanzab]

Here's a nice post of Flow's new error formatting: https://medium.com/flow-type/better-flow-error-messages-for-the-javascript-ecosystem-73b6da948ae2

[brendanzab]

Oh cool. Seems like they are hashing things out with the LSP folks on microsoft/language-server-protocol#166 as well. 🤔

[brendanzab]

Some nice output from Hedgehog hedgehogqa/fsharp-hedgehog#80 (comment)