Improve error handling

[takahi-i]

• accessing 'text' field in higher.py
• ranking of equals.py

Traceback (most recent call last):
  File "/usr/local/bin/esqa", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1128, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1053, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1659, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 1395, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python3.9/site-packages/click/core.py", line 754, in invoke
    return __callback(*args, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/esqa/cli.py", line 41, in assertion
    results = runner.run(config=load(config), index_name=index)
  File "/usr/local/lib/python3.9/site-packages/esqa/validator.py", line 40, in run
    errors.extend(self.check(case, index_name))
  File "/usr/local/lib/python3.9/site-packages/esqa/validator.py", line 45, in check
    return self._check(search_results, case)
  File "/usr/local/lib/python3.9/site-packages/esqa/validator.py", line 51, in _check
    errors.extend(a.validate(es_results=search_results, case_name=case.name))
  File "/usr/local/lib/python3.9/site-packages/esqa/asserts/higher.py", line 27, in validate
    message=f'[{case_name}] Document with {self.item.field} = {self.item.value} ({candidate["text"]}) is ranked lower than specified {self.rank} ({i}-th ranked).',
KeyError: 'text'