Handle conversion errors gracefully in fair-z3, detect bad ADTs

src/main/scala/leon/solvers/ADTManager.scala

src/main/scala/leon/solvers/smtlib/SMTLIBSolver.scala

src/main/scala/leon/solvers/z3/AbstractZ3Solver.scala

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

src/main/scala/leon/solvers/z3/UninterpretedZ3Solver.scala