apycot #2040717 Make updating to unkown branch a warning not an error [waiting feedback]

Some simple projects/dependencies don't have a stable/default branch, so when apycot tries to 'hg update on stable' it fails, and this appears as an error, I think it could be considered as a warning ...

prioritynormal
typebug
done in<not specified>
load0.500
load left0.500
closed by<not specified>