Allow 3rd party extensions to track errors in language client & server

[testforstephen]

Track the language client & server errors to improve the reliability of the product.

[rgrunber]

Looking at it, I think this is fine, so feel free to merge. The only thing I can think of is that there are a few other places where error message may be encountered :

vscode-java/src/extension.ts

Line 244 in 75dc796

initializationFailedHandler: error => {

(the return true case from above might be particularly dangerous as that used to result in a stack overflow for a disposed connection but could be interesting to know how/if the client is failing to initialize the connection)

vscode-java/src/standardLanguageClient.ts

Line 158 in 75dc796

case 'Error':

vscode-java/src/standardLanguageClient.ts

Line 251 in 75dc796

case MessageType.Error:

Are any of these interesting to also report, or would you intend to do that in a separate PR.

[testforstephen]

let's use a separate PR to cover more errors on client side.