Detailed potential errors in the watch function.

src/lib.rs

@@ -122,6 +122,11 @@ impl Hotwatch {
  /// Note that handlers will be run in hotwatch's watch thread, so you'll have to use `move`
  /// if the closure captures anything.
  ///
+ /// # Errors
+ ///
+ /// Watching will fail if the path can't be read, thus returning
+ /// a `hotwatch::Error::Io(std::io::Error)`.
+ ///
  /// # Examples
  ///
  /// ```