Fix the build of timezone.rs on recent Rust versions.

Author: sunfishcode

cap-time-ext/src/timezone.rs

@@ -4,9 +4,18 @@ use iana_time_zone::get_timezone;
 /// A reference to a timezone resource.
 pub struct Timezone(());
 
+/// An error type returned by `Timezone::timezone_name`.
 #[derive(Debug)]
 pub struct TimezoneError(String);
 
+impl std::error::Error for TimezoneError {}
+
+impl std::fmt::Display for TimezoneError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 impl Timezone {
     /// Constructs a new instance of `Self`.
     ///