additional checks for delayed scope retrieval

[pfitzseb]

This prevents a LS crash with certain JMD files (see #280), but doesn't fix the cause of the issue.

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[ZacLN]

I'd prefer to fix the root issue as I think there are other areas where I make the assumption that a scope can be retrieved from any given EXPR... but I can't replicate this on my machine (with the code in the attached issue) so happy for this to go in for now

[pfitzseb]

I could repro that issue locally, but only in a jmd context, so when processed via https://github.com/julia-vscode/LanguageServer.jl/blob/5284d8f3778906d7d5d0ccb96b884403f35b387d/src/requests/textdocument.jl#L247

[ZacLN]

Ah - I think it wasn't working without the file being saved, it seems to be crashing now

[ZacLN]

I think julia-vscode/LanguageServer.jl#935 should fix it - could you check?