Define WeightedBranches as default cost model

Improve error output by specifying which one is the default.

src/main/scala/leon/synthesis/CostModel.scala

@@ -25,6 +25,8 @@ abstract class CostModel(val name: String) {
 }
 
 object CostModel {
+  def default: CostModel = WeightedBranchesCostModel
+
   def all: Set[CostModel] = Set(
     NaiveCostModel,
     WeightedBranchesCostModel

src/main/scala/leon/synthesis/SynthesisPhase.scala

@@ -48,9 +48,12 @@ object SynthesisPhase extends LeonPhase[Program, Program] {
             options = options.copy(costModel = model)
           case None =>
 
-            val errorMsg = "Unknown cost model: " + cm + "\n" +
-                           "Defined cost models: \n" +
-                           (CostModel.all.map(_.name).mkString("  - ", "\n  - ", ""))
+            var errorMsg = "Unknown cost model: " + cm + "\n" +
+                           "Defined cost models: \n"
+
+            for (cm <- CostModel.all.toSeq.sortBy(_.name)) {
+              errorMsg += " - " + cm.name + (if(cm == CostModel.default) " (default)" else "") + "\n"
+            }
 
             ctx.reporter.fatalError(errorMsg)
         }

src/main/scala/leon/synthesis/SynthesizerOptions.scala

@@ -7,5 +7,5 @@ case class SynthesizerOptions(
   searchWorkers: Int               = 1,
   firstOnly: Boolean               = false,
   timeoutMs: Option[Long]          = None,
-  costModel: CostModel             = NaiveCostModel
+  costModel: CostModel             = CostModel.default
 )