[Vimode]Improve presentation of unset buffers on error

Prevent the "Nothing in register x" error linger.

This can be illustrated by trying to paste from an empty buffer in vimode (e.g. "zp) or by setting 2 marks fast enough (e.g. in vimode, qqmambq@q@q)

For now, this can only be practically illustrated using the 1st method as 2 KTextEditor::Messages are always posted when pasting from an empty buffer (one from modebase.cpp and one from insertvimode.cpp)

Before

After

The other minor change is that a-z buffers are now always shown in lowercase