Update error logging for bad patterns in hexdoc (#931)

Author: object-Object

doc/src/hexdoc_hexcasting/book/page/pages.py

@@ -19,7 +19,10 @@ def patterns(self) -> list[PatternInfo]:
     @model_validator(mode="after")
     def _post_root_lookup(self, info: ValidationInfo):
         hex_ctx = HexContext.of(info)
-        self._patterns = [hex_ctx.patterns[self.op_id]]
+        pattern = hex_ctx.patterns.get(self.op_id)
+        if pattern is None:
+            raise ValueError(f"Unknown pattern ID (check your pattern stubs in hexdoc.toml): {self.op_id}")
+        self._patterns = [pattern]
         return self
 
     @model_validator(mode="after")