--out replaces the previous --format option
Currently --in can only take 'hurl' format,
but it will include other formats such as curl or har in the future.
Runtime errors are tested by error hurl files (error_*.hurl),
while export to JSON/HTML is tested by *.json and *.html files.
The error files do add more syntax/functionalities than the standard/non-error hurl files,
therefore, it is not useful to test (and maintain) their JSON/HTML export.
