add fexec error reporting

bootstrap/src/exec.rs

@@ -246,6 +246,10 @@ pub fn main() -> ! {
         &extrainfo,
         None,
     )
+    .map_err(|e| {
+        let _ = libredox::call::write(1, alloc::format!("fexec_impl failed: {}\n", e).as_bytes());
+        e
+    })
     .expect("failed to execute init");
 
     // According to elf(5), PT_INTERP requires that the interpreter path be