sync: make TryAcquireError public (#3250)

The [`Semaphore::try_acquire`][1] method currently returns a private error type.

[1]: https://docs.rs/tokio/0.3/tokio/sync/struct.Semaphore.html#method.try_acquire

tokio/src/sync/mod.rs

@@ -444,7 +444,7 @@ cfg_sync! {
 
     pub(crate) mod batch_semaphore;
     mod semaphore;
-    pub use semaphore::{Semaphore, SemaphorePermit, OwnedSemaphorePermit};
+    pub use semaphore::{Semaphore, SemaphorePermit, OwnedSemaphorePermit, TryAcquireError};
 
     mod rwlock;
     pub use rwlock::{RwLock, RwLockReadGuard, RwLockWriteGuard};