`InternalServerError: no such file or directory` when Downloading file with using File Locker.

Describe the bug

When I try to get file in subdirectory, tusd returns 500 response.

To Reproduce Steps to reproduce the behavior:

Make your tusd server with http://127.0.0.1:8080/basedir
curl 127.0.0.1:8080/basedir/some/your/sub-directory/file.jpeg
See error: msg=InternalServerError message="open <Eliminated>/some/your/sub-directory/file.jpeg.lock.1967546860: no such file or directory"

Expected behavior

404 Not Found should be right

Setup details Please provide following details, if applicable to your situation:

Operating System: macOS (but may be in other systems too)
Used tusd version: v2.4.0
Used tusd data storage: disk store (also disk lock)
Used tusd configuration: my custom golang code. and It's not important on this situation.
Used tus client library: No. I just used curl.

func (handler *UnroutedHandler) GetFile(http.ResponseWriter, *http.Request) {
    // ...
    if handler.composer.UsesLocker {
        lock, err := handler.lockUpload(c, id) // <-- (1)
        if err != nil {
            handler.sendError(c, err)
            return
        }

        defer lock.Unlock()
    }
    // ...
}

func (handler *UnroutedHandler) lockUpload(c *httpContext, id string) (Lock, error) {
    // ...
    if err := lock.Lock(ctx, releaseLock); err != nil { // <-- (2)
        return nil, err // <-- (3) *fs.PathError (ENOENT)
    }
    // ...
}

I've tried to open PR, but It looks uncertain where to handle this error.
- In lockfile, It doesn't seems to proper to handle "no file".
- In func lockUpload(), the lock is abstracted, so it also doesn't look like proper to handle "no file" too.

tus / tusd

`InternalServerError: no such file or directory` when Downloading file with using File Locker. #1148