Move UI code for showing errors out of `custom_rule.py`.

[dlyongemallo]

Incidentally, make the warning dialogs modal (see issue #156).

[dlyongemallo]

Two further questions:

• check_rule returns a bool, but this is never used. Is this return value necessary?
• check_rule is always called with show_error = True. Is this parameter necessary?

[jvdwetering]

I think the bool was originally intended to check if the operation succeeded. But since with this commit it instead raises an Exception, that no longer makes sense. Also with this behaviour, supressing the exceptions with show_error also makes less sense so that that parameter can be removed.

[dlyongemallo]

Okay, I've removed both the parameter and the return value.