[wasm] Refactor encoder.h to use a proper buffer and remove OldFunctions section.

src/wasm/wasm-result.cc

Index: src/wasm/wasm-result.cc
diff --git a/src/wasm/wasm-result.cc b/src/wasm/wasm-result.cc
index 3de5812e9f47638753246775a6ce60cca637c672..08d8f06406c66ed350ec01fc7e0cdebeb2f64ae6 100644
--- a/src/wasm/wasm-result.cc
+++ b/src/wasm/wasm-result.cc
@@ -29,7 +29,8 @@ std::ostream& operator<<(std::ostream& os, const ErrorCode& error_code) {
 }
 
 void ErrorThrower::Error(const char* format, ...) {
-  if (error_) return;  // only report the first error.
+  // only report the first error.
+  if (error_ || isolate_->has_pending_exception()) return;
   error_ = true;
   char buffer[256];