use exit code 16 on exit with validation errors per bids apps execution spec

Author: rwblair

src/bids-validator.ts

@@ -4,5 +4,5 @@ const result = await main()
 
 const errors = result.issues.get({ severity: 'error' })
 if (errors.length) {
-  Deno.exit(1)
+  Deno.exit(16)
 }