Ringleader fails if the tool root cert has already been included in a different category

[psiinon]

This may sound unusual but I've actually seen it happen a couple of times. Users manually add the ZAP root cert as 'Servers' cert (maybe others as well?) which means it wont work properly and Firefox then prevents it from being added as an 'Authorities' cert. If you use PnH then it seems to get in a loop :/ There may also be problems if people manually add the cert but dont check the box to trust it for websites. The cleanest option might be to try deleting the cert first? Or do we need tow warn the user that they've added it incorrectly?