Revert backup chunk specification for suppressing useless chunk backup sets

"Fluent::Unrecoverable Error" make the log set backup per a whole chunk unit, not only relevant log message.
It cause irrelevant logs lost due to whole chunk backup, in other words, not sending and backup whole chunk data set including the bigger log than 256 kb.

In addition the chunk backup file is remained as useless things, because there is no solution to restore the backup sets now.
Refer fluent/fluentd#3813

Author: bysnupy

lib/fluent/plugin/out_cloudwatch_logs.rb

@@ -7,8 +7,6 @@ module Fluent::Plugin
   class CloudwatchLogsOutput < Output
     Fluent::Plugin.register_output('cloudwatch_logs', self)
 
-    class TooLargeEventError < Fluent::UnrecoverableError; end
-
     helpers :compat_parameters, :inject, :formatter
 
     DEFAULT_BUFFER_TYPE = "memory"
@@ -379,7 +377,8 @@ def put_events_by_chunk(group_name, stream_name, events)
       while event = events.shift
         event_bytesize = event[:message].bytesize + EVENT_HEADER_SIZE
         if MAX_EVENT_SIZE < event_bytesize
-          raise TooLargeEventError, "Log event in #{group_name} is discarded because it is too large: #{event_bytesize} bytes exceeds limit of #{MAX_EVENT_SIZE}"
+          log.warn "Log event in #{group_name} is discarded because it is too large: #{event_bytesize} bytes exceeds limit of #{MAX_EVENT_SIZE}"
+          break
         end
 
         new_chunk = chunk + [event]