Add extra line to error message for clarification.

gitflow-sh-setup

@@ -53,7 +53,7 @@ gitflow_require_branch() {
 gitflow_require_branch_absent() {
 	echo "$ALL_BRANCHES" | grep "^$1\$" 2>/dev/null >/dev/null
 	if [ $? -eq 0 ]; then
-		die "Branch '$1' already exists."
+		die "Branch '$1' already exists. Pick another name."
 	fi
 }