[mypyc] Improve error message for multiple inheritance.

mypyc/mypyc#962

Author: jab

mypyc/irbuild/prepare.py

@@ -301,7 +301,12 @@ def prepare_class_def(
     # Set up the parent class
     bases = [mapper.type_to_ir[base.type] for base in info.bases if base.type in mapper.type_to_ir]
     if not all(c.is_trait for c in bases[1:]):
-        errors.error("Non-trait bases must appear first in parent list", path, cdef.line)
+        errors.error(
+            "Multiple inheritance is not currently supported (except for traits)."
+            " Ref: https://mypyc.rtfd.io/en/latest/native_classes.html#inheritance",
+            path,
+            cdef.line,
+        )
     ir.traits = [c for c in bases if c.is_trait]
 
     mro = []