[#210] Refactor integrational testing engine and its interface

Description

Problem: The way the testing engine works (keep going even a contract interpretation error occurs, use Either+Validated to check the engine state) is in stark contrast with the standard way of writing testing frameworks in Haskell (errors are thrown in MonadError/MonadThrow, execution stops unless caught with catchError/catch). Over time this has proven to be an inconvenience.

Solution:

• Removed Validated, IntegrationalValidator, SuccessValidator.
• Removed expectAnySuccess
• Make use of MonadError to signal/inspect contract interpretation errors.
• For simplification, validate was removed from the public interface

Related issue(s)

Resolves #210 (closed)

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