Exit with a non-zero status on error

With `ls` from Debian coreutils 8.26-2

```
ls /bad/path
echo $? # => 2
```

Reproduced same behaviour with exa

Fix ogham#135

src/bin/main.rs

@@ -10,13 +10,18 @@ fn main() {
     let mut stdout = stdout();
 
     match Exa::new(&args, &mut stdout) {
-        Ok(mut exa) => if let Err(e) = exa.run() {
-            match e.kind() {
-                ErrorKind::BrokenPipe => exit(0),
-                _ => {
-                    writeln!(stderr(), "{}", e).unwrap();
-                    exit(1);
-                },
+        Ok(mut exa) => {
+            match exa.run() {
+                Ok(exit_status) => exit(exit_status),
+                Err(e) => {
+                    match e.kind() {
+                        ErrorKind::BrokenPipe => exit(0),
+                        _ => {
+                            writeln!(stderr(), "{}", e).unwrap();
+                            exit(1);
+                        },
+                    };
+                }
             };
         },
         Err(e) => {

src/exa.rs

@@ -60,9 +60,10 @@ impl<'w, W: Write + 'w> Exa<'w, W> {
         })
     }
 
-    pub fn run(&mut self) -> IOResult<()> {
+    pub fn run(&mut self) -> IOResult<i32> {
         let mut files = Vec::new();
         let mut dirs = Vec::new();
+        let mut exit_status = 0;
 
         // List the current directory by default, like ls.
         if self.args.is_empty() {
@@ -72,6 +73,7 @@ impl<'w, W: Write + 'w> Exa<'w, W> {
         for file_name in self.args.iter() {
             match File::from_path(Path::new(&file_name), None) {
                 Err(e) => {
+                    exit_status = 2;
                     try!(writeln!(stderr(), "{}: {}", file_name, e));
                 },
                 Ok(f) => {
@@ -98,10 +100,10 @@ impl<'w, W: Write + 'w> Exa<'w, W> {
         self.options.filter.filter_argument_files(&mut files);
         try!(self.print_files(None, files));
 
-        self.print_dirs(dirs, no_files, is_only_dir)
+        self.print_dirs(dirs, no_files, is_only_dir, exit_status)
     }
 
-    fn print_dirs(&mut self, dir_files: Vec<Dir>, mut first: bool, is_only_dir: bool) -> IOResult<()> {
+    fn print_dirs(&mut self, dir_files: Vec<Dir>, mut first: bool, is_only_dir: bool, exit_status: i32) -> IOResult<i32> {
         for dir in dir_files {
 
             // Put a gap between directories, or between the list of files and
@@ -141,15 +143,18 @@ impl<'w, W: Write + 'w> Exa<'w, W> {
                     }
 
                     try!(self.print_files(Some(&dir), children));
-                    try!(self.print_dirs(child_dirs, false, false));
+                    match self.print_dirs(child_dirs, false, false, exit_status) {
+                        Ok(_) => (),
+                        Err(e) => return Err(e),
+                    }
                     continue;
                 }
             }
 
             try!(self.print_files(Some(&dir), children));
         }
 
-        Ok(())
+        Ok(exit_status)
     }
 
     /// Prints the list of files using whichever view is selected.