Report error on initialize better

[razzeee]

These should report either via popup or as a diagnostic.
#488 (comment)

[jmbockhorst]

Specifically, here (unparseable elm.json):

elm-language-server/src/elmWorkspace.ts

Line 719 in 003ede1

return JSON.parse(await this.host.readFile(elmJsonPath)) as ElmJson;

and here (unsolvable dependencies):

elm-language-server/src/elmWorkspace.ts

Line 507 in 003ede1

throw new Error("Problem getting package version");

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.