“Crumbs!” is a small British cry of surprise, like “oh!” or “goodness!” You might hear “Oh crumbs, I forgot my keys.” It is mild and a little old-fashioned: grandparents, children’s books, and gentle comedy use it more than young people in the street. It is not rude. It is almost always its own little sentence, often with “oh” in front. Americans almost never use it this way. You already know crumbs as the tiny bits of bread on a plate — that is a different use, the ordinary plural of crumb. This surprise crumbs is probably a polite stand-in for a stronger cry, though it looks the same as the bread word.