✨ feat(error): introduce custom error enum for cull-gmail

- define `Error` enum to encapsulate errors from google_gmail1 and google_clis_common
- use `thiserror` crate for simplified error handling

src/error.rs

@@ -0,0 +1,12 @@
+use thiserror::Error;
+
+/// Error messages for cull-gmail
+#[derive(Debug, Error)]
+pub enum Error {
+    /// Error from the google_gmail1 crate
+    #[error("Google Gmail1 says: {0}")]
+    GoogleGmail1(#[from] google_gmail1::Error),
+    /// Error from the google_clis_common crate
+    #[error("Google CLIs Common says: {0}")]
+    InvalidOptionsError(google_clis_common::CLIError, i16),
+}