variable-resolver-service should let clients know if variables cannot be resolved

[elaihau]

VariableResolverService.resolve() returns undefined if a variable is not defined in Theia. And therefore its clients need extra work to differentiate the following 2 scenarios:

1. the variable is "resolved as undefined"
2. the variable is not defined

It would be helpful if such support is added to variable-resolver-service.

This is a follow-up issue suggested in #6331 (comment)

[akosyakov]

We should rather treat undefined as impossible value in APIs, i.e. it should not be ever possible to return by resolvers, but a resolver should be able throw an error instead. We should also review each resolver to align error messages and timing with VS Code. Both tasks and debug extensions should be tested.