Proper handling of cache errors

[slrtbtfs]

Currently, errors that are raised when accessing the cache aren't properly forwarded to the language client.

For a more correct error handling, all the occurrences in the langserver package of stuff like

promql-langserver/langserver/completion.go

Lines 35 to 38 in a45e11c

	location, err := s.cache.Find(&params.TextDocumentPositionParams)
	if err != nil {
	return nil, nil
	}

should be replaced by something like

 location, err := s.cache.Find(&params.TextDocumentPositionParams) 
 if err != nil { 
 	return nil, filterContextExpirations(err) 
 } 

where filterContextExpirations(err) should return nil if the error was caused by an expired document context (in this case the document has changed since the request has started processing and the request is cancelled) and err otherwise.