[eval] Better error message for process spawning error

```haxe
class EvalSpawn {
public static function main():Void {
for (i in 0...100) {
var proc = new sys.io.Process("sleep", ["0"]);
proc.exitCode();
}
}
}
```

The above results in:

```
Uncaught exception process_run
/haxe/std/eval/_std/sys/io/Process.hx:1: character 1 : Called from here
EvalSpawn.hx:4: characters 15-49 : Called from here
```

Adding `proc.close()` fixes this of course, but the error message is not really helpful. Could `eval` detect it cannot spawn another process and throw a "resource exhaustion" (or similar) exception?