better error handling when annotating multiple slices

[JochenGehrig]

when trying to do multi slice annotation on a multichannel 2D image I get the following message follow by console errors. Not that it is very meaningful to do, so I guess you just want to not allow that case. That is fine! But error handling should be clearer. Multicolor 3D seems to work.

[sankeert1999]

yaa i get it the message shoud had been clearer now it's confusing. What i meant to do here is that whenever there is a case such that it's a 3D stack (xyc) i expect user to go through single slice annotation where they specify the channel number in which they want to anotate. I guess what would be better is to directly push it to single slice annoation so that user doesn't need to worry about that

[sankeert1999]

Fixed it by not prompting the user to select the annoation mode