Support multiple exit codes

..with more specific error enums.

cli/src/errors.rs

@@ -0,0 +1,59 @@
+use taskchampion::Error as TcError;
+use thiserror::Error;
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("Command-Line Syntax Error: {0}")]
+    Arguments(String),
+
+    #[error(transparent)]
+    TaskChampion(#[from] TcError),
+
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}
+
+impl Error {
+    /// Construct a new command-line argument error
+    pub(crate) fn for_arguments<S: ToString>(msg: S) -> Self {
+        Error::Arguments(msg.to_string())
+    }
+
+    /// Determine the exit status for this error, as documented.
+    pub fn exit_status(&self) -> i32 {
+        match *self {
+            Error::Arguments(_) => 3,
+            _ => 1,
+        }
+    }
+}
+
+impl From<std::io::Error> for Error {
+    fn from(err: std::io::Error) -> Self {
+        let err: anyhow::Error = err.into();
+        Error::Other(err)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+    use anyhow::anyhow;
+
+    #[test]
+    fn test_exit_status() {
+        let mut err: Error;
+
+        err = anyhow!("uhoh").into();
+        assert_eq!(err.exit_status(), 1);
+
+        err = Error::Arguments("uhoh".to_string());
+        assert_eq!(err.exit_status(), 3);
+
+        err = std::io::Error::last_os_error().into();
+        assert_eq!(err.exit_status(), 1);
+
+        err = TcError::Database("uhoh".to_string()).into();
+        assert_eq!(err.exit_status(), 1);
+    }
+}