Add Fluent namespace to ConfigError in v0.14 API based plugins

[repeatedly]

One user reports following error:

2016-09-09 09:01:49 +0000 [error]: fluent/supervisor.rb:567:rescue in main_process: unexpected error error="uninitialized constant Fluent::Plugin::GrepFilter::ConfigError

[tagomoris]

👍 - but, it's better to add tests to assert_raised for these configuration errors.

[repeatedly]

but, it's better to add tests to assert_raised for these configuration errors.

I agree but this changes several plugins across plugin types.
I think separating PR is good for me.
We have a room for test improvement, use data instead of each and more.

[tagomoris]

Ah, you're right.