Change span of as_dyn_error() to point compile error at attribute.

tests/ui/transparent-struct-not-error.rs

@@ -0,0 +1,9 @@
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+#[error(transparent)]
+pub struct Error {
+    message: String,
+}
+
+fn main() {}