parser_json: fix wrong LoadError warning

[daipom]

Which issue(s) this PR fixes:
None

What this PR does / why we need it:
If Oj is not installed, LoadError with the empty message is raised.

• fluentd/lib/fluent/plugin/parser_json.rb

  Line 53 in b25e989

  raise LoadError unless Fluent::OjOptions.available?

So, the current condition /\boj\z/.match?(ex.message) does not work and the following meaningless warning is displayed.

{datetime} [warn]: #x {id} LoadError

After this fix, the log message will be:

{datetime} [info]: #x {id} Oj is not installed, and failing back to Yajl for json parser

Docs Changes:
Not needed.

Release Note:
Same as the title.

[ashie]

It seems that this rescue no longer needed.
It would be better to do retry at when :oj instead of rescue block.

[daipom]

It seems that this rescue no longer needed. It would be better to do retry at when :oj instead of rescue block.

Thanks for your review!
I have fixed this as https://github.com/fluent/fluentd/compare/151e6314a371add407f602d50cb8be777109afd1..d515b3a6a277d854378a5961223b4666fb438c31

[daipom]

Fixed typo: https://github.com/fluent/fluentd/compare/29ff3815c38bb3f36be0b64a9c27ecae3cdbf11a..891ce71120198fa168232a796d4267a3cdb2c91f

[daipom]

It looks like failing back should be falling back.
For now, this PR does not handle it.
(Changing log messages should not be done casually. For this level, it might be better to leave the existing.)

[ashie]

Thanks!

[daipom]

Thanks for your review!