give human readable error messages if a connector or pin is not available

litex/build/generic_platform.py

@@ -167,7 +167,8 @@ class ConnectorManager:
                     raise ValueError(f"\"{identifier}\" {err}") from err
                 if pn.isdigit():
                     pn = int(pn)
-
+                assert conn in self.connector_table, f"No connector named '{conn}' is available"
+                assert pn in self.connector_table[conn], f"There is no pin '{pn}' on connector '{conn}'"
                 conn_pn = self.connector_table[conn][pn]
                 if ":" in conn_pn:
                     conn_pn = self.resolve_identifiers([conn_pn])[0]