rpk/api: Return back friendly errors

[c4milo]

when positional arguments are missing in all subcommands of rpk api. Instead of
Cobra's generic errors. Fixes half of #300.

Before

$ rpk api topic create
Error: accepts 1 arg(s), received 0

After

$ rpk api topic create
Error: topic's name is missing.

Usage:
  rpk api topic create <topic name> [flags]

Flags:
      --compact            Enable topic compaction
  -h, --help               help for create
  -p, --partitions int32   Number of partitions (default 1)
  -r, --replicas int16     Replication factor. If it's negative or is left unspecified, it will use the cluster's default topic replication factor. (default -1)

Global Flags:
      --brokers strings   Comma-separated list of broker ip:port pairs
      --config string     Redpanda config file, if not set the file will be searched for in the default locations
  -v, --verbose           enable verbose logging (default false)

[emaxerrno]

@c4milo really great idea! thank you.

looks like some simple test strings need to be updated. waiting for the go mod patch to finish building and I'll merge that too.

[c4milo]

@0x5d, @senior7515, this is ready for review. FWIW, I got caught up by topic_test.go trailing spaces, my IDE trims them by default and I had to restore them to get the tablewriter tests passing again 😬

[c4milo]

Because we are using Cobra's RunE function, any errors returned from it will make Cobra to automatically print the Usage help, even if the error has nothing to do with CLI usage. We may want to use Cobra differently after GA, to have more flexibility on CLI UX.

[c4milo]

@c4milo really great idea! thank you.

@senior7515, it was @rkruze idea :)

[emaxerrno]

LGTM