Sometimes users would get confused because an option they were looking
for was not mentioned on a subcommand's man page, and they had not
noticed that the main git-annex man page had a list of common options.
This change lets each subcommand mention the common options, similarly
to how the matching options are handled.
This commit was sponsored by Svenne Krap on Patreon.
Added --json-error-messages option, which includes error messages in the
json output, rather than outputting them to stderr.
The actual rediretion of errors is not implemented yet, this is only
the docs and option plumbing.
This commit was supported by the NSF-funded DataLad project.
