Add a bar color for multiple errors in a different buffer

If you prevent jumps with 'J' you'd still want to be informed about
multiple errors.

Change the gui background color to DarkMagenta to make the text better
visible.  Also, be explicit about the gui attribute.
INTERFACE: Rename different-buffer error-bar

Reflect single error property in the highlight group which was
previously used for all errors in different buffers.

Rename to MakeGreenDifferentBufferOneErrorBar
Add a flag to always use the two main error bars

The new flag 'm' always displays errors with the main two error bars
even for target errors in different buffers.

In combination with `J`, this allows you to use the different-buffer
bars exclusively as a jump notification.  For example, if you execute
tests from the SUT without jumping you already know that the error will
be in a different buffer.