Output json_last_error_msg instead of json_last_error

[jkrzefski]

The JsonErrorException already converts the error code from json_last_error into a human readable error message. The RuntimeException however does not do this. To have more consistency between the two and for improved human readability I have changed json_last_error to json_last_error_msg. Error messages now no longer look like this:

[Elasticsearch\Common\Exceptions\RuntimeException]  
Failed to JSON encode: 5

And instead they look more like this:

[Elasticsearch\Common\Exceptions\RuntimeException]                               
Failed to JSON encode: Malformed UTF-8 characters, possibly incorrectly encoded

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

💚 CLA has been signed

[jkrzefski]

I signed the CLA now. How can I trigger the CLA checker service again?

[ezimuel]

@jkrzefski thanks for this PR. I checked and the CLA is ok. I'm checking about the Travis CI failing. The PR seems good to me.

[ezimuel]

@jkrzefski I see the issue. You need to send the PR against 7.x and not master. I suggest to rebase from 7.x and send a new PR. Thanks!

I just added a Contributing section about that in the README.

[jkrzefski]

@ezimuel I rebased my changes and changed the target branch. Will I have to file a new pull request or is this also ok?

[ezimuel]

@jkrzefski thanks everything is ok now.

[ezimuel]

Just released elasticsearch-php 7.9.0 with #1045 included.