SaltStack: Is there a way to only display failed and warnings in output?

Change the state_output in master's configuration file.

It is also possible to override the state output from the command line, like:

salt '*' state.apply --state-output=mixed

Take a look at the documentation for more information about the state-output.

I usually use:

salt '*' state.apply -l quiet --state-verbose=false

that shows only change/failed states

You can use JSON output, then filter out the successful entries with jq.

salt-run jobs.lookup_jid 20180318whatever --out=json | \
jq 'map_values(with_entries(select(.value.result|not)))'