Add Result alias (#271)

Adds a `yubikey::Result` alias with `yubikey::Error` as the error type.

Since we only have one `Error` type, this simplifies the return types
where a `Result` is returned.

cli/src/commands.rs

@@ -36,7 +36,7 @@ pub struct YubiKeyCli {
 
 impl YubiKeyCli {
     /// Print usage information
-    pub fn print_usage() -> Result<(), io::Error> {
+    pub fn print_usage() -> io::Result<()> {
         let mut stdout = STDOUT.lock();
         stdout.reset()?;