Improved config handling and bugfixes

[AnomalRoil]

This PR is doing a few changes to the way we handle the Config action, its logging and also improves a few comments.

This is notably fixing #2673 by properly handling all the different cases for a local vs global config change not necessarily leading to a git add and git commit in the store.
Sadly, there is no way when using the gitconfig Set command to know if the option was written to the global vs local config, and so to do this really nicely we would need to change the signature of the func (c *Config) Set(mount, key, value string) error function to include a more explicit return value.
But we can fix the problem by properly handling the different errors than can arise during the Add and Commit phases of the config, which is what this PR does.

This PR also prevents us from setting the global config while reporting a misleading "No existing configuration found" error.
Previously, upon using gopass config some.name true on an uninitialized store, we would get the following:

🌟 Welcome to gopass!
⚠ No existing configuration found.
☝ Please run 'gopass setup'
true

Now we get just the expected true, since we do support changing the global config without having a store. When trying to do it on a local config, tho, it will now fail:

• for the root store with:

   $ gopass config --store "<root>" some.name true
   
   Error: Store <root> seems uninitialized or cannot be initialized
  

• on a substore, with:

   $ gopass config --store "test" some.name true
   Error: Error setting config value: failed to set config value "some.name": substore 'test' is not initialized or doesn't exist
  

[AnomalRoil]

While doing this, I realized we are not properly supporting worktree level configs in gopass:
a worktree level config will take precedence over a local config or a global config, but we cannot alter or set its values from within gopass.

Is that expected @dominikschulz ?

[dominikschulz]

My goal was to support worktree configs, but it was rather low priority and at this point I don't remember if I finished that part or not.

But the documentation claims that I didn't complete it: https://pkg.go.dev/github.com/gopasspw/gopass/pkg/gitconfig#hdr-Known_limitations

[dominikschulz]

Very nice! A few more remarks and questions, but I think we're almost there.

[dominikschulz]

LGTM. Thanks!