Raise an error when attempting to bulk-index without any corpora

[bartier]

This PR attempts to solve the issue #568.

As discussed in the issue, now the BulkIndexParamSource constructor checks whether that self.corpora is empty and if it is raise exceptions.InvalidSyntax.

Summary of changes:

• Explicit error is raised when list of corpora is empty and you try to bulk requests to Elasticsearch.
  
  Output example when running a Rally race without corpora definition.

• Fix tests that started to fail after the new error check.

• Add new test for this new error check.

    def test_create_without_corpora_definition(self):
        with self.assertRaises(exceptions.InvalidSyntax) as ctx:
            params.BulkIndexParamSource(track=track.Track(name="unit-test"), params={})

        self.assertEqual("There is no document corpora definition for track unit-test. "
                         "You must add at least one before making bulk requests to Elasticsearch.", ctx.exception.args[0])

I hope this helps :)

[danielmitterdorfer]

@elasticmachine test this please

[danielmitterdorfer]

Great change! I left one grammar-related nit but apart from that the change looks good. Thank you. :)

[bartier]

@danielmitterdorfer I made the requested changes. Could you check again? BTW, I found a small typo in runner.py, I think it would be okay to add to this PR :)

[danielmitterdorfer]

@elasticmachine test this please

[danielmitterdorfer]

Thanks for your PR and thanks for iterating! The change looks good now and I'll merge it as soon as CI passes.