improved logging when pipeline terminates by error

[jsvd]

What does this PR do?

Expose the exceptions when a pipeline terminates, either from a worker exception or plugin register error.

Why is it important/What is the impact to the user?

Users with errors in their pipelines must enable the debug flag to know why their pipeline isn't working.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

How to test this PR locally

Verify that a pipeline logs the cause of its termination, when the error happens:

1. at plugin register time (e.g. raise exception in ruby filter's init setting)
2. pipeline validation time (e.g. start pipeline with syntax error or missing a plugin's required option)
3. when a worker crashes (e.g. edit a filter plugin to raise exception in filter(events))

Related issues

fixes #12232

Use cases and Logs

Before:

❯ docker run --rm -it -e XPACK_MONITORING_ENABLED=false logstash:7.9.1 bin/logstash -e "filter { ruby { init => 'throw(\"hey\")' code => ''} }" --log.level=warn
2020/09/29 08:45:49 Setting 'xpack.monitoring.enabled' from environment.
Sending Logstash logs to /usr/share/logstash/logs which is now configured via log4j2.properties
[2020-09-29T08:46:07,880][WARN ][logstash.config.source.multilocal] Ignoring the 'pipelines.yml' file because modules or command line options are specified
[2020-09-29T08:46:10,359][ERROR][logstash.agent           ] Failed to execute action {:id=>:main, :action_type=>LogStash::ConvergeResult::FailedAction, :message=>"Could not execute action: PipelineAction::Create<main>, action_result: false", :backtrace=>nil}
[2020-09-29T08:46:15,560][ERROR][org.logstash.Logstash    ] java.lang.IllegalStateException: Logstash stopped processing because of an error: (SystemExit) exit

After:

❯ bin/logstash -e "filter { ruby { init => 'throw(\"hey\")' code => ''} }" --log.level=warn 
Using JAVA_HOME defined java: /Users/joaoduarte/.jenv/versions/11.0
Sending Logstash logs to /Users/joaoduarte/elastic/logstash/logs which is now configured via log4j2.properties
[2020-09-29T09:45:14,438][WARN ][logstash.config.source.multilocal] Ignoring the 'pipelines.yml' file because modules or command line options are specified
[2020-09-29T09:45:15,951][ERROR][logstash.javapipeline    ][main] Pipeline terminated by error {:pipeline_id=>"main", :exception=>UncaughtThrowError, :message=>"uncaught throw \"hey\"", :backtrace=>["org/jruby/RubyKernel.java:1204:in `throw'", "(ruby filter init):1:in `register'", "org/jruby/RubyKernel.java:1048:in `eval'", "/Users/joaoduarte/elastic/logstash/vendor/bundle/jruby/2.5.0/gems/logstash-filter-ruby-3.1.5/lib/logstash/filters/ruby.rb:58:in `register'", "org/logstash/config/ir/compiler/AbstractFilterDelegatorExt.java:75:in `register'", "/Users/joaoduarte/elastic/logstash/logstash-core/lib/logstash/java_pipeline.rb:231:in `block in register_plugins'", "org/jruby/RubyArray.java:1809:in `each'", "/Users/joaoduarte/elastic/logstash/logstash-core/lib/logstash/java_pipeline.rb:230:in `register_plugins'", "/Users/joaoduarte/elastic/logstash/logstash-core/lib/logstash/java_pipeline.rb:565:in `maybe_setup_out_plugins'", "/Users/joaoduarte/elastic/logstash/logstash-core/lib/logstash/java_pipeline.rb:243:in `start_workers'", "/Users/joaoduarte/elastic/logstash/logstash-core/lib/logstash/java_pipeline.rb:188:in `run'", "/Users/joaoduarte/elastic/logstash/logstash-core/lib/logstash/java_pipeline.rb:137:in `block in start'"], "pipeline.sources"=>["config string"], :thread=>"#<Thread:0x34caad83 run>"}
[2020-09-29T09:45:15,984][ERROR][logstash.agent           ] Failed to execute action {:pipeline_id=>:main, :action=>"Create", :message=>"Could not execute action: PipelineAction::Create<main>, action_result: false", :backtrace=>nil}
[2020-09-29T09:45:21,262][ERROR][org.logstash.Logstash    ] java.lang.IllegalStateException: Logstash stopped processing because of an error: (SystemExit) exit