Saving a file in the repo while it is preparing causes an error

[carolynvs]

1. Save a file in the repo.
2. Note that deconst starts preparing.
3. Save another file in the repo.
4. This causes an error as deconst tries to kick off the preparation stage while one is already running, causing a container name conflict.

Error: HTTP code is 409 which indicates error: undefined - Conflict.
The name "preparer-content-1" is already in use by container 7e21c3632016.
You have to remove (or rename) that container to be able to reuse that name.

Most times I can just wait and retry, I don't need to restart the client like in deconst/deconst-docs#167.

It would be nice if deconst checked if it was currently running, and either skip kicking off another prepare or wait until the current one is complete and start another.