Jump to error in an open window containing it, if possible.

I work with 2 windows open, one with the tests, one with the code.

When I run the tests when in the code window I don't like the way that :make changed the buffer of the code-window to the buffer of the tests, while it is already available in another window. So I end up with 2 windows both with the tests open and having to switch buffers back to the code.

This patch makes the jump to the already open window if available, and falls back to the old bufferswitch behaviour if that window is not available.

If there is a smarter way to fix this outside of makegreen.vim, I'd love to know that.