Function read_source 

pub fn read_source(
    vault: &dyn Vault,
    file: &Path,
) -> Result<String, MarkdownError>

Reads a markdown source through vault by its index key file.

The error names the key, as the file-based readers name the path, and keeps the I/O kind (crate::vault::VaultError’s conversion), so a missing file is still NotFound.