Don't unwrap the result of span_to_snippet

It can return `Err` due to macros being expanded across crates or
files.

src/test/ui/borrowck/move-error-snippets.rs

@@ -0,0 +1,23 @@
+// Test that we don't ICE after trying to construct a cross-file snippet #63800.
+
+// compile-flags: --test
+
+#[macro_use]
+#[path = "move-error-snippets-ext.rs"]
+mod move_error_snippets_ext;
+
+struct A;
+
+macro_rules! sss {
+    () => {
+        #[test]
+        fn fff() {
+            static D: A = A;
+            aaa!(D);         //~ ERROR cannot move
+        }
+    };
+}
+
+sss!();
+
+fn main() {}