Implement `Display` trait for `ManticoreError`

[oooo-ps]

I'm currently using this construction because the Anyhow crate can't handle ManticoreError type:

client
    .sql(
        "CREATE TABLE IF NOT EXISTS ... (
            ...
        )",
        Some(true),
    )
    .await
    .map_err(|e: ManticoreError| {
        Error::msg(match e {
            ManticoreError::Api(e) => todo!(),
            ManticoreError::Header(e) => e.to_string(),
            ManticoreError::Http(e) => e.to_string(),
            ManticoreError::Hyper(e) => e.to_string(),
            ManticoreError::HyperClient(e) => e.to_string(),
            ManticoreError::Serde(e) => e.to_string(),
            ManticoreError::UriError(e) => e.to_string(),
        })
    })?;