Disambiguate evaluation Errors/Failures

src/main/scala/leon/evaluators/CodeGenEvaluator.scala

src/main/scala/leon/evaluators/DefaultEvaluator.scala

src/main/scala/leon/evaluators/EvaluationResults.scala

src/main/scala/leon/evaluators/Evaluator.scala

src/main/scala/leon/solvers/RandomSolver.scala

src/main/scala/leon/solvers/z3/FairZ3Solver.scala

src/main/scala/leon/solvers/z3/FunctionTemplate.scala

src/main/scala/leon/synthesis/LinearEquations.scala

src/main/scala/leon/synthesis/rules/Cegis.scala

src/test/scala/leon/test/evaluators/EvaluatorsTests.scala

src/test/scala/leon/test/purescala/LikelyEq.scala