Nicer error message if missing configuration

[rubiojr]

Wraps the error to give a bit more context, from this:

❯ gh slack auth Error: could not find key "extensions"

To:

❯ gh slack auth Error: failed to read gh-slack configuration: could not find key "extensions"

[rneatherway]

Oh nice, thanks! Is it also possible to include the path that it tries to load it from?

[rneatherway]

I've had a go at that and the error message now reads like:

Error: failed to read gh-slack configuration value "extensions.slack.team" from "/Users/rneatherway/.config/gh/config.yml": could not find key "slack"

What do you think of that?

[rubiojr]

Reads great, thanks for taking care of that!