Only check for ENOSPC on inotify_add_watch

According to the inotify documentation, the only function that returns
ENOSPC is `inotify_add_watch`, so this moves the check for this error
code to the `inotify.add_watch()` call point. This makes sure someone
using a different watcher like PollWatcher on linux won't reinterpret
this error.

Also, this uses `libc::ENOSPC` instead of `28` because it's easier to
interpret from the inotify documentation.

src/error.rs

@@ -78,11 +78,6 @@ impl Error {
 
     /// Creates a new i/o Error from a stdlib `io::Error`.
     pub fn io(err: io::Error) -> Self {
-        // do not report inotify limits as "no more space" on linux #266
-        #[cfg(target_os = "linux")]
-        if err.raw_os_error() == Some(28) {
-            return Self::new(ErrorKind::MaxFilesWatch);
-        }
         Self::new(ErrorKind::Io(err))
     }
 

src/inotify.rs

@@ -481,7 +481,14 @@ impl EventLoop {
 
         if let Some(ref mut inotify) = self.inotify {
             match inotify.add_watch(&path, watchmask) {
-                Err(e) => Err(Error::io(e)),
+                Err(e) => {
+                    // do not report inotify limits as "no more space" on linux #266
+                    if err.raw_os_error() == Some(libc::ENOSPC) {
+                        Err(Error::new(ErrorKind::MaxFilesWatch))
+                    } else {
+                        Err(Error::io(e))
+                    }
+                }
                 Ok(w) => {
                     watchmask.remove(WatchMask::MASK_ADD);
                     self.watches