add fmt::Error as core Error source

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/error.rs

@@ -85,6 +85,8 @@ pub enum Error {
 	#[error("{0}")]
 	AdminCommand(&'static str),
 	#[error("{0}")]
+	Fmt(#[from] fmt::Error),
+	#[error("{0}")]
 	Err(String),
 }