git-gui: Cleaned up error message formatting.

git-gui: Cleaned up error message formatting.

Added an extra blank line between the first line of each error message
and the rest of the message, as usually the rest of the message is
coming from Tcl or is the stderr output of a git command we tried to
invoke.  This makes it easier to read the output (if any).

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