Avoid panicking when inputting error app name; List all apps when initializing.

os/src/loader.rs

@@ -51,3 +51,12 @@ fn app_names() -> Vec<&'static str> {
     }
     v
 }
+
+pub fn list_apps() {
+    let apps = app_names();
+    println!("/**** APPS ****");
+    for app in apps {
+        println!("{}", app);
+    }
+    println!("**************/")
+}

os/src/main.rs

@@ -46,6 +46,7 @@ pub fn rust_main() -> ! {
     trap::init();
     trap::enable_timer_interrupt();
     timer::set_next_trigger();
+    loader::list_apps();
     task::run_tasks();
     panic!("Unreachable in rust_main!");
 }

os/src/syscall/process.rs

@@ -59,10 +59,13 @@ pub fn sys_fork() -> isize {
 pub fn sys_exec(path: *const u8) -> isize {
     let token = current_user_token();
     let path = translated_str(token, path);
-    let data = get_app_data_by_name(path.as_str()).unwrap();
-    let task = current_task().unwrap();
-    task.exec(data);
-    0
+    if let Some(data) = get_app_data_by_name(path.as_str()) {
+        let task = current_task().unwrap();
+        task.exec(data);
+        0
+    } else {
+        -1
+    }
 }
 
 pub fn sys_waitpid(pid: isize, exit_code_ptr: *mut i32) -> isize {