Error handling for trailing slash in project subdirectory

[emmahunt]

Is your feature request related to a problem? Please describe.

The provider allows you to create a project where the subdirectory setting has a trailing / e.g. dbt/. This is not allowed in the UI (screenshot), so should not be allowed in the provider

Describe the solution you'd like

Error handling in the TF provider (e.g. a message when unsupported value is provided) to prevent a trailing /

Describe alternatives you've considered

Additional context