feat(usage): add ability to get usage string for subcommands too

You can now get the usage even for sub-commands by calling the
usage() method.

Breaking Change
ArgMatches::usage() now returns a slice (no longer an Option<&str>)
This is to improve ergonomics, as there should always be at least a
default usage statement, there should never be None

src/app.rs

@@ -706,7 +706,6 @@ impl<'a, 'v, 'ab, 'u, 'ar> App<'a, 'v, 'ab, 'u, 'ar>{
                 }
             }
         }
-        matches.usage = Some(self.create_usage());
         self.get_matches_from(&mut matches, &mut it );
 
         matches
@@ -879,7 +878,7 @@ impl<'a, 'v, 'ab, 'u, 'ar> App<'a, 'v, 'ab, 'u, 'ar>{
                         }
                     }
                 } else {
-                    self.report_error(format!("Positional argument \"{}\" was found, but {} wasn't expecting any", arg, self.name), true, true);
+                    self.report_error(format!("Argument \"{}\" isn't a valid argument for {}", arg, self.bin_name.clone().unwrap_or(self.name.clone())), true, true);
                 }
             }
         }
@@ -899,6 +898,7 @@ impl<'a, 'v, 'ab, 'u, 'ar> App<'a, 'v, 'ab, 'u, 'ar>{
                     true, true);
         }
 
+        matches.usage = Some(self.create_usage());
 
         if let Some(sc_name) = subcmd_name {
             if let Some(ref mut sc) = self.subcommands.get_mut(&sc_name) {

src/args/argmatches.rs

@@ -266,7 +266,7 @@ impl<'a> ArgMatches<'a> {
         ("", None)
     }
 
-    /// Returns a slice of the default usage for the *top level parent App only*
+    /// Returns a slice of the usage
     ///
     ///
     /// # Example
@@ -276,10 +276,12 @@ impl<'a> ArgMatches<'a> {
     /// # let app_matches = App::new("myapp").subcommand(SubCommand::new("test")).get_matches();
     /// println!("{}",app_matches.usage().unwrap());
     /// ```
-    pub fn usage(&self) -> Option<&str> {
+    pub fn usage(&self) -> &str {
         if let Some( ref u ) = self.usage {
-            return Some(&u[..]);
+            return &u[..];
         }
-        None
+
+        // Should be un-reachable
+        ""
     }
 }