REPL eval error from crash reporting

[davidanthoff]

We have crash reports with this error message:

The handler for the 'repl/runcode' request returned a value of type VSCodeServer.EvalErrorStack, which is not a valid return type according to the request definition.

This means that our handler inside the LS for that request is not always wrapping the return value in a ReplRunCodeRequestReturn. I tried to understand where that might happen, but the whole REPL execution framework is too complicated for me to understand right now :) @pfitzseb, this is probably for you.

[xgdgsc]

During development of #3123 , I found when https://github.com/julia-vscode/julia-vscode/pull/3123/files#diff-78600594f1b69fa4e2f8213a0a3cf1aa6ebdaf7006b729337ea5e8c649ef11bdR261 anything inside savePersistentResults goes wrong it would return EvalErrorStack. So there could be some unhandled error around.