Fix empty LetDef's, tidy up CEGIS, better CEGIS errors

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/purescala/ExprOps.scala

src/main/scala/leon/purescala/Expressions.scala

src/main/scala/leon/purescala/ScopeSimplifier.scala

src/main/scala/leon/purescala/TypeOps.scala

src/main/scala/leon/solvers/SolverUnsupportedError.scala

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

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

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

src/main/scala/leon/utils/UnitElimination.scala

src/main/scala/leon/xlang/ImperativeCodeElimination.scala