[TSServer] missing tsconfig.json(s) included in the msbuild content list impacts LS features

[jramsay]

If a tsconfig.json is present in the list of msbuild content files but the file doesn't actually exist TSServer throws a missing file exception and LS features stop working.

Discussed with @vladima - we need to add a check to see if the tsconfig.json file actually exists before adding it to the list of tsConfigFiles in openExternalProjects.