Change the error when trying to write to a file to EROFS

src/main.rs

@@ -37,7 +37,7 @@ impl fs_rpc::Server for Serv {
     fn open(&self, path: &std::path::Path, mode: FileOpenMode, mount_id: u64) -> Result<(Option<u64>, u64), Errno> {
         if !mode.readable() || mode.writable() {
             // WRiting isn't supported yet, so the only sensible mode is readonly.
-            return Err(Errno::EACCES);
+            return Err(Errno::EROFS);
         }
         let mounts = self.mounts.read();
         let mount = &mounts[mount_id as usize];