throwing exception if we cannot print jsvalue

libjsengine/JSEngine.h

@@ -21,12 +21,16 @@
  */
 
 #pragma once
+#include <exception>
 
 namespace dev
 {
 namespace eth
 {
 
+class JSException: public std::exception {};
+class JSPrintException: public JSException { char const* what() const noexcept { return "Cannot print expression!"; } };
+
 class JSString
 {
 public:

libjsengine/JSV8Engine.cpp

@@ -35,7 +35,9 @@ namespace eth
 
 static char const* toCString(v8::String::Utf8Value const& _value)
 {
-	return *_value ? *_value : "<string conversion failed>";
+	if (*_value)
+		return *_value;
+	throw JSPrintException();
 }
 
 // from:        https://github.com/v8/v8-git-mirror/blob/master/samples/shell.cc

libjsengine/JSV8Printer.cpp

@@ -43,5 +43,7 @@ JSString JSV8Printer::prettyPrint(JSV8Value const& _value) const
 	v8::Local<v8::Value> values[1] = {v8::Local<v8::Value>::New(_value.value())};
 	v8::Local<v8::Value> res = func->Call(func, 1, values);
 	v8::String::Utf8Value str(res);
-	return *str ? *str : "<pretty print conversion failed>";
+	if (*str)
+		return *str;
+	throw JSPrintException();
 }