Just store bones that could not get renamed to desired flipped name on the
first try into a temp list, and try to rename them a second time.
This is rather simple solution, will induce 'over numbering' in case you
flip a bone to another unselected bone's name (since number will be
incremented in both rename attempts), but think this is acceptable minor
glitch, for a corner case situation that does not have any good
resolution anyway.
Also, set `strip_numbers` option of `BKE_deform_flip_side_name` to
false, otherwise chains of bones with same names would get their numbers
completely messed up after name flipping.
Based on work by @dfelinto in D2456 (https://developer.blender.org/D2456), thanks.
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