git-gui: Use native tk_messageBox for errors.

git-gui: Use native tk_messageBox for errors.

Rather than drawing our own toplevel for error messages we really
should just use the the native tk_messageBox command to display
any error messages.

Major benefits for doing so are:
  - automatically centers over main window;
  - less code required on our part in git-gui;
  - includes a nifty error icon on most systems;
  - better fits the look-and-feel of the operating system.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>