Don't show CLI usage on API error

[mickael-menu]

This disables CLI usage output when an error occurs while parsing a publication (See spf13/cobra#340).

Basically changes:

$ rwp manifest invalid.epub
Error: failed opening invalid.epub: failed parsing asset: failed loading container.xml: resource: error 404: couldn't find /META-INF/container.xml in FileFetcher paths
Usage:
  rwp manifest <pub-path> [flags]

Flags:
  -h, --help                help for manifest
  -i, --indent string       Indentation used to pretty-print
      --infer-a11y string   Infer accessibility metadata: no, merged, split (default "no")
      --infer-page-count    Infer the number of pages from the generated position list.

to just:

$ rwp manifest invalid.epub
Error: failed opening invalid.epub: failed parsing asset: failed loading container.xml: resource: error 404: couldn't find /META-INF/container.xml in FileFetcher paths

[chocolatkey]

LGTM