Report error on initialize better

[razzeee]

These should report either via popup or as a diagnostic. https://github.com/elm-tooling/elm-language-server/pull/488#issue-560799485

[jmbockhorst]

Specifically, here (unparseable elm.json): https://github.com/elm-tooling/elm-language-server/blob/003ede17fc99ae22fd04f5bdcbc059f2c14d1a98/src/elmWorkspace.ts#L719

and here (unsolvable dependencies): https://github.com/elm-tooling/elm-language-server/blob/003ede17fc99ae22fd04f5bdcbc059f2c14d1a98/src/elmWorkspace.ts#L507

are 2 places we can report diagnostics in the elm.json

[razzeee]

Unparsable elm.json actually seems to be handled quite nicely via the json schema we defined. But this only helps for vscode.