Crash of type-checker when a module is not found

[jurgenvinju]

Describe the bug

See https://github.com/usethesource/rascal-language-servers/issues/281

Such an unfound module should lead to a lucid error message (including perhaps the offending pathConfig), instead of a hard crash.

The effect downstream is that silently none of the Rascal IDE features based on the checker work.