sync: improve error message when synchronization root parent directory is missing

[Doow501]

Problem

When creating or resuming a sync session against an SSH endpoint whose
synchronization root parent directory does not exist, Mutagen reports an error
like:

<root>: unable to walk to transition root parent: unable to open synchronization root parent directory: no such file or directory

This message is technically correct, but it is not very actionable for users.
It took me a while to understand that the real problem was that the parent
directory of the synchronization root did not exist on the remote side.

Example

For example, with:

• alpha: /Users/.../Workspace/MyAIProject/src
• beta: gpu_1only:/data/kk/MyAIProject/src

if /data/kk/MyAIProject does not exist remotely, the session shows
that it is connected, but also reports the transition-root-parent error and
cannot synchronize.

Expected behavior

It would be much easier to diagnose if the error made it explicit that the
parent directory of the synchronization root is missing on the remote side.

Something along the lines of:

remote synchronization root parent directory does not exist

or even including the relevant path if available.

Possible improvement

One of these would help:

1. Improve the error message to clearly say that the synchronization root parent
   directory is missing.
2. Add documentation stating that the parent directory of the synchronization
   root must already exist and be accessible.

Motivation

This seems like a relatively small change, but it would make first-time setup
much easier to debug, especially for SSH-based remote development workflows.

I’d be happy to work on a PR if this direction makes sense.

[Doow501]

Thanks. After looking into the code path more closely, I think there may be a small behavior improvement worth considering in addition to the error-message improvement.

Today, Mutagen already seems willing to create a missing synchronization root during transition, as long as the root's parent directory already exists. The failure happens when the parent hierarchy itself is missing, because transition can't walk to the root parent.

So the possible behavior change I had in mind is narrower than "always create the root automatically":

• only for writable beta endpoints
• only if the synchronization root itself does not already exist
• only create the missing parent directory hierarchy of the root
• do not eagerly create the root leaf itself
• preserve clear errors if parent creation fails

That would effectively provide mkdir -p semantics for the root's parent chain, while still letting the existing transition logic create the root itself.

If you'd prefer to keep the current behavior and only improve the error message/documentation, that's also fine. I mainly wanted to check whether this narrower behavior change would be considered acceptable before preparing a PR.