Handling of invalid tasks

[skodapetr]

As of now it is possible to manually start prediction with invalid chains or PDB/uniport code (see #52). There should be a way how to handle this. The main issue is that before the execution is started this information is not available.

As downloading the structure and reading chains may take a few seconds it is also not possible to do this with the current UI - we may need to, at least, add a waiting dialog.

While the current approach provide user with an error, it will create the prediction directory on the server which is undesirable.