Allow specifying `gardenctl logs --error` for getting only error level logs

[dansible]

It would be great if we can specify different verbosity levels for gardenctl logs for each component (info|warn|error) so that only those logs are shown for each sub component

[dansible]

At a minimum it would be great to be able to parse just the error messages. Some logs use the format of starting error messages with E... and some arbitrary numbers in the start of the log line, others include the full error keyword