Convert Exception during converge to a failed action (elastic#13969)

Certain errors during pipeline converge will emit an IllegalStateException - for example attempting
to reference an environment variable that does not exist. Attempting to add a java exception to the
converge result would result in an uncaught exception in a pipeline thread leading to an unclean
shutdown.

This had the effect of,prior to this commit, Logstash behaviour varying depending on the class of
error that caused a pipeline not to start - an invalid pipeline configuration would still enable
logstash to start other pipelines in a multiple pipeline configuration, or automatic reloading
enabling changes to the configuration to allow the pipeline to start(in multi- and single pipeline
configurations).
However, a missing environment variable would cause Logstash to crash hard no matter what.

This was discovered when updating to jruby-9.3.3.0, when new clean up code joining existing
threads to perform a graceful shutdown was stuck indefinitely, due to the webserver shutdown
code not being called, due to the unclean shutdown.

logstash-core/lib/logstash/agent.rb

@@ -391,7 +391,7 @@ def converge_state(pipeline_actions)
           end
         rescue SystemExit, Exception => e
           logger.error("Failed to execute action", :action => action, :exception => e.class.name, :message => e.message, :backtrace => e.backtrace)
-          converge_result.add(action, e)
+          converge_result.add(action, LogStash::ConvergeResult::FailedAction.from_exception(e))
         end
       end
     end.each(&:join)

logstash-core/spec/logstash/agent_spec.rb

@@ -280,6 +280,13 @@
           expect(json_document["message"]).to eq("foo-bar")
         end
       end
+
+      context "referenced environment variable does not exist" do
+
+        it "does not converge the pipeline" do
+          expect(subject.converge_state_and_update).not_to be_a_successful_converge
+        end
+      end
     end
 
     describe "#upgrade_pipeline" do
@@ -304,6 +311,15 @@
         subject.shutdown
       end
 
+      context "when the upgrade contains a bad environment variable" do
+        let(:new_pipeline_config) { "input { generator {} } filter { if '${NOEXIST}' { mutate { add_tag => 'x' } } } output { }" }
+
+        it "leaves the state untouched" do
+          expect(subject.converge_state_and_update).not_to be_a_successful_converge
+          expect(subject.get_pipeline(default_pipeline_id).config_str).to eq(pipeline_config)
+        end
+      end
+
       context "when the upgrade fails" do
         it "leaves the state untouched" do
           expect(subject.converge_state_and_update).not_to be_a_successful_converge