rhg: Remove error message on unsupported CLI arguments
Like in other "unsupported" cases that return a specific exit code
Differential Revision: https://phab.mercurial-scm.org/D10002
pub type ExitCode = i32;
/// Successful exit
pub const OK: ExitCode = 0;
/// Generic abort
pub const ABORT: ExitCode = 255;
/// Command or feature not implemented by rhg
pub const UNIMPLEMENTED: ExitCode = 252;