An istat_result object is a list with the following components:
success: Logical indicating if operation succeeded
data: data.table with downloaded data (or NULL on failure)
exit_code: Integer exit code (0=success, 1=error, 2=timeout)
message: Character message describing result
md5: Character MD5 checksum of data (or NA if not computed)
is_timeout: Logical indicating if failure was due to timeout
timestamp: POSIXct timestamp when result was created