libgolang: PanicError += what()

Give what() to PanicError so that uncaught panics give proper message on
std::terminate_handler crash instead of printing just "std::exception",
for example

    terminate called after throwing an instance of 'golang::PanicError'
      what():  chan: decref: free: recvq not empty

instead of

    terminate called after throwing an instance of 'golang::PanicError'
      what():  std::exception

golang/runtime/libgolang.cpp

@@ -65,6 +65,10 @@ namespace golang {
 
 struct PanicError : exception {
     const char *arg;
+
+    virtual const char *what() const throw() {
+        return arg;
+    }
 };
 
 // panic throws exception that represents C-level panic.