8.0 (b2019040718)
In 7, I would sometimes check if a tag existed by checking the quality of the tag and checking if it returned 404 / NOT_FOUND. In 8 however, my quality checks for these non-existent tags is returning an error instead and no useable value. Using the try(…) function also doesn’t catch this error and return the error result either 
Expression example:
toStr(tag("folder/tag.quality")) != "Not Found
EDIT: Checking the error being reported on the expression toStr(tag("folder/tag.quality")) I get Error_TypeConversion
In the meantime, I will use runScript to execute the system.tag.exist(…) function.