Minor, FunctionCodegen: add additional logging on missing function descriptor

Trying to diagnose light class building failures

compiler/backend/src/org/jetbrains/kotlin/codegen/FunctionCodegen.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2015 JetBrains s.r.o.
+ * Copyright 2010-2016 JetBrains s.r.o.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -108,8 +108,9 @@ public class FunctionCodegen {
 
     public void gen(@NotNull KtNamedFunction function) {
         SimpleFunctionDescriptor functionDescriptor = bindingContext.get(BindingContext.FUNCTION, function);
-        assert functionDescriptor != null : "No descriptor for function " + function.getText() + "\n" +
-                                            "in " + function.getContainingFile().getVirtualFile();
+        if (functionDescriptor == null) {
+            throw ExceptionLogger.logDescriptorNotFound("No descriptor for function " + function.getName(), function);
+        }
 
         if (owner.getContextKind() != OwnerKind.DEFAULT_IMPLS || function.hasBody()) {
             generateMethod(JvmDeclarationOriginKt.OtherOrigin(function, functionDescriptor), functionDescriptor,