get_printable_location for debugging seems to fail when using '%' string formatting

[samgiles]

get_printable_location in naulang/runtime/executioncontext.py fails when the JIT enters for the test_jit_functioncall test with the following output:

RPython traceback:
  File "naulang_runtime_executioncontext.c", line 4948, in get_printable_location
  File "rpython_rtyper_lltypesystem_rdict.c", line 162, in ll_dict_getitem__dicttablePtr_Signed
Fatal RPython error: KeyError

This seems to go away when the string formatting is replaced with something like return "". This is probably an RPython bug.