Fix JSON_USE_EXCEPTION=0 use case

This patch fixes the JSON_USE_EXCEPTION flag. Currently, due to the
throwRuntimeError and throwLogicError methods implemented in json_value,
even if JSON_USE_EXCEPTION is set to 0 jsoncpp will still throw. This
breaks integration into projects with -fno-exceptions set, such as
Chromium.

include/json/value.h

@@ -50,6 +50,7 @@
  */
 namespace Json {
 
+#if JSON_USE_EXCEPTION
 /** Base class for all exceptions we throw.
  *
  * We use nothing but these internally. Of course, STL can throw others.
@@ -85,6 +86,7 @@ class JSON_API LogicError : public Exception {
 public:
   LogicError(String const& msg);
 };
+#endif
 
 /// used internally
 JSONCPP_NORETURN void throwRuntimeError(String const& msg);