core: show warning when failed to detect out node in non trivial switch

See [issue #38](https://github.com/skylot/jadx/issues/38)
Actually 'toString()' methods will be called automatically, this is main reason (don't make anything if this message will not output because of logger filter level) of using '{}' construction.
```java
LOG.warn("Can't detect out node for switch block: {} in {}", block, mth);
```