Use `'<unknown>'` for unresolvable `.boxed_type_str`

Add a teensie unit test to match.

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/test_reg_err_types.py

@@ -165,6 +165,16 @@ class TestRegErrTypesPlumbing:
             is tractor.ContextCancelled
         )
 
+    def test_boxed_type_str_without_ipc_msg(self):
+        '''
+        When a `RemoteActorError` is constructed
+        without an IPC msg (and no resolvable type),
+        `.boxed_type_str` should return `'<unknown>'`.
+
+        '''
+        rae = RemoteActorError('test')
+        assert rae.boxed_type_str == '<unknown>'
+
 
 # -- IPC-level integration tests --
 

tractor/_exceptions.py

@@ -491,7 +491,7 @@ class RemoteActorError(Exception):
         if self._ipc_msg:
             return self._ipc_msg.boxed_type_str
 
-        return ''
+        return '<unknown>'
 
     @property
     def boxed_type(self) -> Type[BaseException]: