Running a jar file silently ignores --sources

If a JAR file is run together with a `--sources` option, then the source files are silently ignored. For example:

```
$ jbang run --sources HttpUser.java jarGAV  config.json
```

Either the source files should be compiled and added to the classpath, or a warning should be displayed informing the JBang user that in this particular context source files are ignored.