[asm.js] Improve error reporting of internal failures.

R=sigurds@chromium.org

Change-Id: I54df6ab72a3d6836be15d4a26b8a1615628eb872
Reviewed-on: https://chromium-review.googlesource.com/1163703Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54935}

src/asmjs/asm-js.cc

@@ -392,8 +392,14 @@ MaybeHandle<Object> AsmJs::InstantiateAsmWasm(Isolate* isolate,
     // An exception caused by the module start function will be set as pending
     // and bypass the {ErrorThrower}, this happens in case of a stack overflow.
     if (isolate->has_pending_exception()) isolate->clear_pending_exception();
+    if (thrower.error()) {
+      ScopedVector<char> error_reason(100);
+      SNPrintF(error_reason, "Internal wasm failure: %s", thrower.error_msg());
+      ReportInstantiationFailure(script, position, error_reason.start());
+    } else {
+      ReportInstantiationFailure(script, position, "Internal wasm failure");
+    }
     thrower.Reset();  // Ensure exceptions do not propagate.
-    ReportInstantiationFailure(script, position, "Internal wasm failure");
     return MaybeHandle<Object>();
   }
   DCHECK(!thrower.error());

src/wasm/wasm-result.h

@@ -123,6 +123,7 @@ class V8_EXPORT_PRIVATE ErrorThrower {
 
   bool error() const { return error_type_ != kNone; }
   bool wasm_error() { return error_type_ >= kFirstWasmError; }
+  const char* error_msg() { return error_msg_.c_str(); }
 
   Isolate* isolate() const { return isolate_; }