Inconsistent service errors when working on non-existing projects

[lorenzo-cavazzi]

Describe the bug
When working with service APIs requiring a project URL, and the provided URL doesn't refer to an existing project, the first time we get a reasonable error (e.g. UserRepoUrlInvalidError), but the following invocations return different errors (e.g. UserNonRenkuProjectError or even IntermittentTimeoutError).

To Reproduce
Try with any API, like GET /config.show providing a non-existing git_url as parameter.

Expected behavior
The error should always be the same, probably UserRepoUrlInvalidError or another invalid repository error.

Additional context
It seems that the cached folder of the project isn't cleaned up properly. The second time the API is invoked, the same folder is checked and the error changes for that reason. The folder is usually empty, usually with a <project_name>.lock file and occasionally also a <something>semapthore.00.lock file.

Probably, cache.project_clone should remove the folder if cloning fails because of a non-existing repo/URL.