[tags] [bug] rename tag to an existing tag causes database error

steps to repeat

tag a photo with 'a'

tag a photo with 'b'

if b is a variant of a, and you want to condense the tags so both are tagged with 'a', goto manage tags, select 'b' from the dropdown and click edit

type 'a' in the box and click save

DATABASE ERROR

this case needs to be handled in a better way, either by mapping all items that are tagged with b to a and deleting 'b', or simply giving the user a friendly error message.

if there is enough interest i'd be willing to work on a fix. as a workaround you can view all items that are tagged with 'b', tag them with 'a' and then delete 'b'