Update and show help on missing file for outline subcommand

[jeremyrickard]

Signed-off-by: jeremyrickard [email protected]

What type of PR is this?

/kind bug
/kind cleanup

What this PR does / why we need it:

This PR updates the document outline command to show help when no file is provided. It also updates the usage string to show the required positional argument:

./bom document outline
bom document outline → Draw structure of a SPDX document",

This subcommand draws a tree-like outline to help the user visualize
the structure of the bom. Even when an SBOM represents a graph structure,
drawing a tree helps a lot to understand what is contained in the document.

You can define a level of depth to limit the expansion of the entities.
For example set --depth=1 to only visualize only the files and packages
attached directly to the root of the document.

bom will try to add useful information to the oultine but, if needed, you can
set the --spdx-ids to only output the IDs of the entities.

Usage:
  bom document outline [SPDX File To Draw] [flags]

Flags:
  -d, --depth int   recursion level (default -1)
  -h, --help        help for outline
      --spdx-ids    use SPDX identifiers in tree nodes instead of names

Global Flags:
      --log-level string   the logging verbosity, either 'panic', 'fatal', 'error', 'warning', 'info', 'debug', 'trace' (default "info")
FATA You should only specify one file

This makes the outline command behave in a consistent way with the generate command for a more consistent user experience.

Which issue(s) this PR fixes:

Fixes: #53

Special notes for your reviewer:

Does this PR introduce a user-facing change?

It updates the user facing error message to indicate that a file is required in the help text and displays usage, in addition to the existing error message.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jeremyrickard

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[puerco]

Yay thanks @jeremyrickard, much better !!
/lgtm