[inspector] provide more usefull error message for non serializable value

src/inspector/StringUtil.cpp

Index: src/inspector/StringUtil.cpp
diff --git a/src/inspector/StringUtil.cpp b/src/inspector/StringUtil.cpp
index 04b9e85c8f471566541cc27f5fb0d7b2f46e9fa1..b1aefc04d8825c17b58872bc7fb4f3da91d813f9 100644
--- a/src/inspector/StringUtil.cpp
+++ b/src/inspector/StringUtil.cpp
@@ -103,7 +103,8 @@ std::unique_ptr<protocol::Value> parseJSON(const String16& string) {
 
 }  // namespace protocol
 
-std::unique_ptr<protocol::Value> toProtocolValue(v8::Local<v8::Context> context,
+std::unique_ptr<protocol::Value> toProtocolValue(protocol::String* errorString,
+                                                 v8::Local<v8::Context> context,
                                                  v8::Local<v8::Value> value,
                                                  int maxDepth) {
   if (value.IsEmpty()) {
@@ -111,7 +112,10 @@ std::unique_ptr<protocol::Value> toProtocolValue(v8::Local<v8::Context> context,
     return nullptr;
   }
 
-  if (!maxDepth) return nullptr;
+  if (!maxDepth) {
+    *errorString = "Object reference chain is too long";
+    return nullptr;
+  }
   maxDepth--;
 
   if (value->IsNull() || value->IsUndefined()) return protocol::Value::null();
@@ -134,9 +138,12 @@ std::unique_ptr<protocol::Value> toProtocolValue(v8::Local<v8::Context> context,
     uint32_t length = array->Length();
     for (uint32_t i = 0; i < length; i++) {
       v8::Local<v8::Value> value;
-      if (!array->Get(context, i).ToLocal(&value)) return nullptr;
+      if (!array->Get(context, i).ToLocal(&value)) {
+        *errorString = "Internal error";
+        return nullptr;
+      }
       std::unique_ptr<protocol::Value> element =
-          toProtocolValue(context, value, maxDepth);
+          toProtocolValue(errorString, context, value, maxDepth);
       if (!element) return nullptr;
       inspectorArray->pushValue(std::move(element));
     }
@@ -147,12 +154,17 @@ std::unique_ptr<protocol::Value> toProtocolValue(v8::Local<v8::Context> context,
         protocol::DictionaryValue::create();
     v8::Local<v8::Object> object = v8::Local<v8::Object>::Cast(value);
     v8::Local<v8::Array> propertyNames;
-    if (!object->GetPropertyNames(context).ToLocal(&propertyNames))
+    if (!object->GetPropertyNames(context).ToLocal(&propertyNames)) {
+      *errorString = "Internal error";
       return nullptr;
+    }
     uint32_t length = propertyNames->Length();
     for (uint32_t i = 0; i < length; i++) {
       v8::Local<v8::Value> name;
-      if (!propertyNames->Get(context, i).ToLocal(&name)) return nullptr;
+      if (!propertyNames->Get(context, i).ToLocal(&name)) {
+        *errorString = "Internal error";
+        return nullptr;
+      }
       // FIXME(yurys): v8::Object should support GetOwnPropertyNames
       if (name->IsString()) {
         v8::Maybe<bool> hasRealNamedProperty = object->HasRealNamedProperty(
@@ -163,16 +175,19 @@ std::unique_ptr<protocol::Value> toProtocolValue(v8::Local<v8::Context> context,
       v8::Local<v8::String> propertyName;
       if (!name->ToString(context).ToLocal(&propertyName)) continue;
       v8::Local<v8::Value> property;
-      if (!object->Get(context, name).ToLocal(&property)) return nullptr;
+      if (!object->Get(context, name).ToLocal(&property)) {
+        *errorString = "Internal error";
+        return nullptr;
+      }
       std::unique_ptr<protocol::Value> propertyValue =
-          toProtocolValue(context, property, maxDepth);
+          toProtocolValue(errorString, context, property, maxDepth);
       if (!propertyValue) return nullptr;
       jsonObject->setValue(toProtocolString(propertyName),
                            std::move(propertyValue));
     }
     return std::move(jsonObject);
   }
-  UNREACHABLE();
+  *errorString = "Object couldn't be returned by value";
   return nullptr;
 }