Set up plugin structure

src/error.rs

@@ -0,0 +1,31 @@
+use std::fmt;
+use std::io;
+
+pub enum Error {
+    Io(io::Error),
+}
+
+impl From<io::Error> for Error {
+    fn from(e: io::Error) -> Self {
+        Error::Io(e)
+    }
+}
+
+// Rust only supports `fn main() -> Result<(), E: Debug>`, so we implement `Debug`
+// manually to provide the error output we want.
+impl fmt::Debug for Error {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        match self {
+            Error::Io(e) => writeln!(f, "Failed to set up YubiKey: {}", e)?,
+        }
+        writeln!(f)?;
+        writeln!(
+            f,
+            "[ Did this not do what you expected? Could an error be more useful? ]"
+        )?;
+        write!(
+            f,
+            "[ Tell us: https://str4d.xyz/age-plugin-yubikey/report              ]"
+        )
+    }
+}