supernova: synthdef corruption - added synthef path to error message

A reproducer to test this:

```
(
s = Server(\test1, NetAddr("localhost",57111) );
v = Server(\test2, NetAddr("localhost",57112) )
)

(
[s,v].do(_.boot)
)

//this will cause synthdef corruption
(
[s,v].do{ |x|
SynthDef(\test, {
	Out.ar(0, SinOsc.ar() )
	}).load(x)
}
)
```