rc git: report error if blame-jump fails

git blame typically fails when the buffile is not tracked by Git.
Let's escalate the failure early instead of continuing witout blame
data, only to eventually hit the generic
"git blame-jump: missing blame info" error.

rc/tools/git.kak

@@ -553,13 +553,22 @@ define-command -params 1.. \
             cursor_line=$3
             cursor_column=$4
             blame_info=$(git blame --porcelain "$starting_commit" -L"$cursor_line,$cursor_line" -- "$file")
+            if [ $? -ne 0 ]; then
+                echo 'echo -markup %{{Error}failed to run git blame, see *debug* buffer}'
+                exit
+            fi
         } else {
             set --
             eval "$prepare_git_blame_args"
             blame_info=$(git blame --porcelain -L"$cursor_line,$cursor_line" "$@" <${contents_fifo})
+            status=$?
             if [ "$contents_fifo" != /dev/null ]; then
                 rm -r $(dirname $contents_fifo)
             fi
+            if [ $status -ne 0 ]; then
+                echo 'echo -markup %{{Error}failed to run git blame, see *debug* buffer}'
+                exit
+            fi
         } fi
         eval "$(printf '%s\n---\n%s' "$blame_index" "$blame_info" |
                 client=${kak_opt_docsclient:-$kak_client} \