Compiler doesn't set exit code with --ksc-json-output
When `--ksc-json-output` is specified, `ksc` no longer fails with error exit code. One need to manually scan for "errors" field on the 2nd level of JSON hierarchy.
```bash
✗ ./kaitai-struct-compiler/bin/kaitai-struct-compiler tests/data/squafs.ksy -t python --ksc-json-output 
{"tests/data/squafs.ksy": {"errors": [{"file": "tests/data/squafs.ksy","message": "tests/data/squafs.ksy (No such file or directory)"}]}}
✗ echo $?
0
```