cobra prints usage on errors

[mattmoor]

Description

After the switch to cobra, we get the usage displayed when a command errors, e.g.

COSIGN_EXPERIMENTAL=true cosign verify ghcr.io/mattmoor/public@sha256:720702b8ffe0a0ea9925db4b3b0a29d1a3d9f669592f3497ffda5091e226523d
No TUF root installed, using embedded CA certificate.
Error: no matching signatures:

Usage:
  cosign verify [flags]

Examples:
  cosign verify --key <key path>|<key url>|<kms uri> <image uri> [<image uri> ...]

  # verify cosign claims and signing certificates on the image
  cosign verify <IMAGE>
...

cc @dekkagaijin @n3wscott (we may want to fix this before 1.3)