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

@@ -27,7 +27,7 b' pub fn run(invocation: &crate::CliInvoca'
         .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"");