Handling of invalid inputs

[aalfons]

There are different ways we could handle errors due to invalid inputs:

1. We could catch them when we click the button (for example to run ROBMED) and then have a pop-up message to instruct users what to change.
2. We could catch invalid inputs on the fly and deactivate the button to prevent such errors in the first place.

Option 1 is easier to implement, but I'm in favor of option 2 because it's cleaner. I think it's less confusing to prevent issues before the user clicks the button. In addition, for the Model tab, we don't have a button to click, but we still need to prevent errors when more than 3 serial mediators are selected (which are not supported, see #31).

What should we do and should we best implement this?

[aalfons]

For 2., we'll need a number of things:

1. We'll need a reactive logical variable (as component of values) that stores if all inputs are valid (so that we can activate the next step in the workflow such as the button to run ROBMED).
2. We'll need an observer that updates this reactive variable (if necessary) whenever one of the relevant inputs changes.
3. We'll need a UI element below each input. If this specific input is invalid, an error message below the input should appear (similar to the warning message when there is no random number seed).

Anything else? Am I missing something?

[AuroreAA]

or maybe just validate and need