rhg: `cat` command: print error messages for missing files
And exit with an error code if no file was matched.
This matches the behavior of Python-based hg.
Differential Revision: https://phab.mercurial-scm.org/D10142
pub type ExitCode = i32;
/// Successful exit
pub const OK: ExitCode = 0;
/// Generic abort
pub const ABORT: ExitCode = 255;
/// Generic something completed but did not succeed
pub const UNSUCCESSFUL: ExitCode = 1;
/// Command or feature not implemented by rhg
pub const UNIMPLEMENTED: ExitCode = 252;