This is to ensure that only the relevant command-line flags
for a certain Hugo subcommand is shown to the end user,
reducing clutter and improving user experience.
Fixes#1624 - CLI UX: Flags shouldn't be global
Cobra, the CLI commander in use in Hugo, has some long awaited improvements in the error handling department.
This enables a more centralized error handling approach.
This commit introduces that by changing all the command funcs to `RunE`:
* The core part of the error logging, usage logging and `os.Exit(-1)` is now performed in one place and that one place only.
* The usage text is now only shown on invalid arguments etc. (user errors)
Fixes#1502
