Error types need to converted into booleans

I noticed that jsonlint return values don’t work properly. Code outside this method treats these values as booleans.

Steps to reproduce:
```
tmp $ cat test.json
{
    "foo" : [ 1,2,3]
}
tmp $ jsonlint test.json
test.json: ok
tmp $ echo $?
0
tmp $ cat test.json
{
    "foo" : [ 1,2,3
}
tmp $ jsonlint test.json
test.json:3:0: Error: Expected a ']' but saw '}'
   |  At line 3, column 0, offset 22
   |  Array started at line 2, column 12, offset 14
test.json: has errors
tmp $ echo $?
0
```

`$?` should be 1, and after applying the attached patch it is.