Add custom exceptions with msg (un)packing

tractor/_exceptions.py

@@ -0,0 +1,50 @@
+"""
+Our classy exception set.
+"""
+import builtins
+import traceback
+
+
+class RemoteActorError(Exception):
+    # TODO: local recontruction of remote exception deats
+    "Remote actor exception bundled locally"
+    def __init__(self, message, type_str, **msgdata):
+        super().__init__(message)
+        self.type = getattr(builtins, type_str, Exception)
+        self.msgdata = msgdata
+
+    # TODO: a trio.MultiError.catch like context manager
+    # for catching underlying remote errors of a particular type
+
+
+class InternalActorError(RemoteActorError):
+    """Remote internal ``tractor`` error indicating
+    failure of some primitive or machinery.
+    """
+
+
+class NoResult(RuntimeError):
+    "No final result is expected for this actor"
+
+
+def pack_error(exc):
+    """Create an "error message" for tranmission over
+    a channel (aka the wire).
+    """
+    return {
+        'error': {
+            'tb_str': traceback.format_exc(),
+            'type_str': type(exc).__name__,
+        }
+    }
+
+
+def unpack_error(msg, chan=None):
+    """Unpack an 'error' message from the wire
+    into a local ``RemoteActorError``.
+    """
+    tb_str = msg['error'].get('tb_str', '')
+    return RemoteActorError(
+        f"{chan.uid}\n" + tb_str,
+        **msg['error'],
+    )