rhg: Fall back to Python on unsupported `rhg config <section>`

Printing an entire section (as opposed to a single value with
`rhg config foo.bar`) is not supported yet in Rust only.

Differential Revision: https://phab.mercurial-scm.org/D10098

rust/rhg/src/commands/config.rs

@@ -1,36 +1,36 b''
 use crate::error::CommandError;
 use clap::Arg;
 use format_bytes::format_bytes;
 use hg::errors::HgError;
 use hg::utils::SliceExt;
 
 pub const HELP_TEXT: &str = "
 With one argument of the form section.name, print just the value of that config item.
 ";
 
 pub fn args() -> clap::App<'static, 'static> {
     clap::SubCommand::with_name("config")
         .arg(
             Arg::with_name("name")
                 .help("the section.name to print")
                 .value_name("NAME")
                 .required(true)
                 .takes_value(true),
         )
         .about(HELP_TEXT)
 }
 
 pub fn run(invocation: &crate::CliInvocation) -> Result<(), CommandError> {
     let (section, name) = invocation
         .subcommand_args
         .value_of("name")
         .expect("missing required CLI argument")
         .as_bytes()
         .split_2(b'.')
-        .ok_or_else(|| HgError::abort(""))?;
+        .ok_or_else(|| HgError::unsupported("hg config <section>"))?;
 
     let value = invocation.config.get(section, name).unwrap_or(b"");
 
     invocation.ui.write_stdout(&format_bytes!(b"{}\n", value))?;
     Ok(())
 }