hg-cpython: fallback when encountering an unknown matcher

At this point in the process, nothing user-visible has happened, it is still
safe to fallback. This can happen now that we're going to be using
"container matchers" like unionmatcher and intersectionmatcher.

This is easier and less error-prone than recursive checking beforehand since
only the presence of a transformation case will allow the process to continue.

rust/hg-cpython/src/dirstate/status.rs

@@ -236,7 +236,7 b' pub fn status_wrapper('
                 after_status,
             )
         }
-        e => Err(PyErr::new::<ValueError, _>(
+        e => Err(PyErr::new::<FallbackError, _>(
             py,
             format!("Unsupported matcher {}", e),
         )),