Tests for failure

[SuperFola]

Is your feature request related to a problem? Please describe. We are currently testing correct behaviour, hit we should also have tests for our errors messages: do they pop up when expected? Are errors correctly reported?

Describe the solution you'd like We will need to refactor the tests to have unit tests in ArkScript ; integration tests in C++ ; "fail tests" with a C++ launcher, ArkScript code and expected output stored in separate files. We need the C++ launcher because errors in ArkScript are unrecoverable by design.

[SuperFola]

#386 will be a requirement for this.