Don't log an ERROR for skipped scheduled executions

[juhoautio]

If a flow is scheduled with concurrentOption=skip, it's perfectly normal that triggering of a schedule is skipped. This PR changes such ERROR lines in the server log to INFO level.

On a general level, in my opinion ERROR level should be only used for platform errors, ie. when Azkaban fails to do something that it promises to be able to do. If this rule holds, it will be easier to monitor that Azkaban is working correctly by checking that server logs don't contain errors.

Before:

2018/10/23 13:41:14.337 +0300 INFO [ExecuteFlowAction] Invoking flow test-project.test-flow
2018/10/23 13:41:14.338 +0300 ERROR [TriggerManager] Failed to do action Execute flow test-flow from project test-project for Trigger Id: 0, Description: Trigger from triggerLoader with trigger condition of ThresholdChecker.eval() and expire condition of EndTimeCheck_1.eval(), Execute flow test-flow from project test-project
java.lang.RuntimeException: azkaban.executor.ExecutorManagerException: Flow is already running. Skipping execution.
	at azkaban.trigger.builtin.ExecuteFlowAction.doAction(ExecuteFlowAction.java:232)
	at azkaban.trigger.TriggerManager$TriggerScannerThread.onTriggerTrigger(TriggerManager.java:363)
	at azkaban.trigger.TriggerManager$TriggerScannerThread.checkAllTriggers(TriggerManager.java:343)
	at azkaban.trigger.TriggerManager$TriggerScannerThread.run(TriggerManager.java:297)
Caused by: azkaban.executor.ExecutorManagerException: Flow is already running. Skipping execution.
	at azkaban.trigger.builtin.ExecuteFlowAction.doAction(ExecuteFlowAction.java:229)
	... 3 more

After:

2018/10/23 13:41:51.778 +0300 INFO [ExecuteFlowAction] Invoking flow test-project.test-flow
2018/10/23 13:41:51.779 +0300 INFO [TriggerManager] Skipped action [Execute flow test-flow from project test-project] for [Trigger Id: 0, Description: Trigger from triggerLoader with trigger condition of ThresholdChecker.eval() and expire condition of EndTimeCheck_1.eval(), Execute flow test-flow from project test-project] because: Flow is already running. Skipping execution.

[codecov]

Codecov Report

Merging #2000 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master    #2000      +/-   ##
============================================
- Coverage     33.09%   33.08%   -0.01%     
+ Complexity     2690     2689       -1     
============================================
  Files           415      415              
  Lines         29280    29280              
  Branches       3714     3715       +1     
============================================
- Hits           9689     9688       -1     
  Misses        18724    18724              
- Partials        867      868       +1

Impacted Files	Coverage Δ	Complexity Δ
...ava/azkaban/trigger/builtin/ExecuteFlowAction.java	44% <0%> (+1.28%)	12 <0> (ø)	⬇️
.../src/main/java/azkaban/trigger/TriggerManager.java	15.31% <0%> (-0.2%)	5 <0> (ø)
...kaban/executor/RunningExecutionsUpdaterThread.java	75.86% <0%> (-3.45%)	6% <0%> (-1%)

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[coveralls]

Pull Request Test Coverage Report for Build 4148

• 0 of 9 (0.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 36.169%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
azkaban-common/src/main/java/azkaban/trigger/builtin/ExecuteFlowAction.java	0	3	0.0%
azkaban-common/src/main/java/azkaban/trigger/TriggerManager.java	0	6	0.0%

Files with Coverage Reduction	New Missed Lines	%
azkaban-common/src/main/java/azkaban/trigger/builtin/ExecuteFlowAction.java	1	49.0%

Totals
Change from base Build 4147:	0.0%
Covered Lines:	10539
Relevant Lines:	29138

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💛 - Coveralls

[burgerkingeater]

looks good! thanks