Error being overridden incorrectly on FS.Rename

[kjpou1]

While renaming a file and if one of the parent paths do not exist the error code is being overwritten to EBUSY instead of reporting the actual error:

Error no 44 Error: ENOENT Message: No such file or directory 

emscripten/src/library_fs.js

Line 729 in 126fa66

throw new FS.ErrnoError({{{ cDefine('EBUSY') }}});

Suggested code that returns back the original error code is the following:

        // parents must exist
        var lookup, old_dir, new_dir;
        try {
          lookup = FS.lookupPath(old_path, { parent: true });
          old_dir = lookup.node;
          lookup = FS.lookupPath(new_path, { parent: true });
          new_dir = lookup.node;
        } catch (e) {
          // do not override failing error
          if (e instanceof FS.ErrnoError)
            throw e;
          throw new FS.ErrnoError(10);
        }

[kripken]

I think that makes sense @kjpou1 , would you like to open a PR?