Replace assert! with an error

Signed-off-by: Jiahao XU <Jiahao_XU@outlook.com>

src/unix.rs

@@ -22,7 +22,9 @@ pub struct Acquired {
 
 impl Client {
     pub fn new(mut limit: usize) -> io::Result<Client> {
-        assert!(limit <= 4096, "Can't have limit larger than 4096 since the pipe would block forever when writing to it");
+        if limit > 4096 {
+            return Err(io::Error::new(io::ErrorKind::Other,  "Can't have limit larger than 4096 since the pipe would block forever when writing to it"));
+        }
 
         let client = unsafe { Client::mk()? };