#245 Make ".canvas lower" work with non-existent belowThis tag

[Steven]

This is new behaviour, and i ~think~ is probably a bug.

If the "belowThis" tag is not defined, wish throws an error.
All previous version os wish this does not happen.

pack [canvas .c]
.c create line 0 0 10 10 -tags aline
.c lower aline noline

> tag "noline" doesn't match any items

[Jan Nijtmans]

No, it's a bug fix: See #3437816

[Donal K. Fellows]

No, it's a FRQ

[Steven]

I't be nice to not throw an error here (and also modify the raise command's behaviour)
* AFAIK - all other references to non-existing tags do not throw errors
* This is the way it has been with "lower" for a long time.

[Steven]

tkspider error

[Steven]

The issue is <b>Many</b> tk apps are now going to needlessly start throwing errors.
My tkspider game does just this. (screenshot attached).

There is no real plus side to this change.