fix corrupted path when passed a path that doesn't contain basePath.

noticed this in some additional usage today.

in most cases, this shouldn't matter but i think its good to handle it. as i ran into another bug with watch logic that was allowing uploads even if not in the shop base. need to figure something out for that for now, i've added a 404 check to the shopify-api to handle with better error message than just "invalid json"