git-branch-list(drop): Show error message when branch not found

Instead of displaying the underlying errors:

    sed: first RE may not be empty
    rm: .git/info/git-branch-list.bak: No such file or directory

display a meaningful error message from our program to let users know
that the given branch couldn't be found in the branch-list database.

1 files changed, 6 insertions, 0 deletions

diff --git a/git-branch-list b/git-branch-list
index 1468bfe..c37ef74 100755
--- a/git-branch-list
+++ b/git-branch-list
@@ -54,12 +54,18 @@ function save_branch () {
 
 function drop_branch () {
 	local branch=$(branch_or_current_branch "$1")
+	local input="$branch"
 
 	# Assume `$branch` is an ID if it's not a branch name
 	if ! is_a_branch "$branch"; then
 		branch=$(branch_by_id "$branch")
 	fi
 
+	if [ -z "$branch" ]; then
+		echo "branch-list: Unrecognised branch '$input'" 1>&2
+		exit 1
+	fi
+
 	sed -i '.bak' "/$branch/d" "$DATABASE"
 	rm "${DATABASE}.bak"
 }