getopts: format failure messages with Show.

This obsoletes the old `to_err_msg` method. Replace

    println!("Error: {}", failure.to_err_msg())

    let string = failure.to_err_msg();

with

    println!("Error: {}", failure)

    let string = failure.to_str();

[breaking-change]

src/librustdoc/lib.rs

@@ -148,7 +148,7 @@ pub fn main_args(args: &[String]) -> int {
     let matches = match getopts::getopts(args.tail(), opts().as_slice()) {
         Ok(m) => m,
         Err(err) => {
-            println!("{}", err.to_err_msg());
+            println!("{}", err);
             return 1;
         }
     };