Fixed Error() instance dumping when "name" prop is not primitive.

Fixed Error() instance dumping when "name" prop is not primitive.

Previously, njs_error_to_string2() might be invoked with error argument
pointing to vm->retval.  When "name" prop was not primitive vm->retval
might be overwritten.  As a result error pointer might be referencing a
primitive value.  In turn the second call of njs_object_property()
received an invalid object pointer because it expects only object value
types.

The fix is to ensure that error object pointer is never overwritten.

This closes #615 issue on Github.