Improve error message: NumberFormatException when checking for an element but only a List is available

https://discuss.elastic.co/t/numberformatexception-stops-logstash-from-working/71973/7

For all general issues, please provide the following details for fast resolution:

- Version:
5.1.2

- Operating System:
Linux chell 4.7.0-0.bpo.1-amd64 #1 SMP Debian 4.7.5-1~bpo8+1 (2016-09-30) x86_64 GNU/Linux

- Config File (if you have sensitive info, please remove it):
```
if [taskExecution][input][bidderId] {
...
}
```

- Sample Data:
This works:
```
{
...
"properties": {
"taskExecution": {
"input": {
"bidderId": 1096,
"changeType": "RulesChanged"
}
}
}
```

This don't:
```
{
...
"properties": {
"taskExecution": {
"input": [
{
"accountId": 527,
"invoiceId": 3259,
"hasInvoiceNumberCounter": true,
"hasChanges": true
}
]
}
}
}
```

- Steps to Reproduce:
Document above not works, because the input contains a List.

```
[2017-01-18T10:56:16,553][DEBUG][logstash.inputs.beats ] config LogStash::Inputs::Beats/@client_inactivity_timeout = 60
deploy@apollo:/etc/logstash$ docker logs logstash-services.1.7ilmsb0gdld6wkpd0j309g6es
[2017-01-18T10:56:25,425][DEBUG][logstash.filters.json ] Running json filter {:event=>2017-01-18T08:16:46.209Z orion {"level":"DEBUG","message":"InvoicesWorkflow/anonymous-step-2: WaitingForExecution","log_source":"smarter.ecommerce.commons.spring.task.ServiceLogTaskRunnerPlugin","properties":{"service":{"name":"Whoop"},"workflowExecution":{"workflowId":"InvoicesWorkflow","id":"InvoicesWorkflow:regenerateInvoice:7b802029-1719-4dfc-ba06-41d45b591347","stepId":"anonymous-step-2"},"taskExecution":{"group":"InvoicesWorkflow","name":"anonymous-step-2","id":"taskRunner://smecTaskRunner/InvoicesWorkflow/anonymous-step-2/8591c1aa-72b7-4cae-a4ed-8dac1d146f49_4","createdAt":1484667736438,"status":"WaitingForExecution","endStatus":null,"priority":100,"startedAt":null,"finishedAt":null,"input":[{"accountId":527,"invoiceId":3259,"hasInvoiceNumberCounter":true,"hasChanges":true}],"lockKey":null,"output":null,"cancellation":null,"exception":null,"duration":null}},"createdAt":1484667736438}}
[2017-01-18T10:56:25,425][DEBUG][logstash.filters.json ] Event after json filter {:event=>2017-01-18T08:16:46.209Z orion InvoicesWorkflow/anonymous-step-2: WaitingForExecution}
[2017-01-18T10:56:25,425][DEBUG][logstash.util.decorators ] filters/LogStash::Filters::Mutate: adding value to field {"field"=>"status", "value"=>["%{[taskExecution][status]}"]}
[2017-01-18T10:56:25,425][DEBUG][logstash.util.decorators ] filters/LogStash::Filters::Mutate: adding value to field {"field"=>"title", "value"=>["%{[workflowExecution][workflowId]} - %{[workflowExecution][stepId]}"]}
[2017-01-18T10:56:25,425][DEBUG][logstash.filters.mutate ] filters/LogStash::Filters::Mutate: removing tag {:tag=>"[taskExecution][exception]"}
...
Exception in pipelineworker, the pipeline stopped processing new events, please check your filter configuration and restart Logstash.
...
Exception in thread "[main]>worker0" java.lang.NumberFormatException: For input string: "bidderId"
at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
at java.lang.Integer.parseInt(Integer.java:580)
at java.lang.Integer.parseInt(Integer.java:615)
at org.logstash.Accessors.fetch(Accessors.java:130)
at org.logstash.Accessors.get(Accessors.java:20)
at org.logstash.Event.getUnconvertedField(Event.java:160)
at org.logstash.ext.JrubyEventExtLibrary$RubyEvent.ruby_get_field(JrubyEventExtLibrary.java:113)
at org.logstash.ext.JrubyEventExtLibrary$RubyEvent$INVOKER$i$1$0$ruby_get_field.call(JrubyEventExtLibrary$RubyEvent$INVOKER$i$1$0$ruby_get_field.gen)
at org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:168)
at org.jruby.ast.CallOneArgNode.interpret(CallOneArgNode.java:57)
at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:105)
at org.jruby.ast.IfNode.interpret(IfNode.java:110)
at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:105)
at org.jruby.ast.BlockNode.interpret(BlockNode.java:71)
at org.jruby.evaluator.ASTInterpreter.INTERPRET_BLOCK(ASTInterpreter.java:112)
at org.jruby.runtime.Interpreted19Block.evalBlockBody(Interpreted19Block.java:206)
at org.jruby.runtime.Interpreted19Block.yield(Interpreted19Block.java:157)
at org.jruby.runtime.Block.yield(Block.java:142)
at org.jruby.RubyArray.eachCommon(RubyArray.java:1606)
at org.jruby.RubyArray.each(RubyArray.java:1613)
at org.jruby.RubyArray$INVOKER$i$0$0$each.call(RubyArray$INVOKER$i$0$0$each.gen)
at org.jruby.runtime.callsite.CachingCallSite.callBlock(CachingCallSite.java:143)
at org.jruby.runtime.callsite.CachingCallSite.callIter(CachingCallSite.java:154)
at org.jruby.ast.CallNoArgBlockNode.interpret(CallNoArgBlockNode.java:64)
at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:105)
at org.jruby.ast.BlockNode.interpret(BlockNode.java:71)
at org.jruby.evaluator.ASTInterpreter.INTERPRET_BLOCK(ASTInterpreter.java:112)
at org.jruby.runtime.Interpreted19Block.evalBlockBody(Interpreted19Block.java:206)
at org.jruby.runtime.Interpreted19Block.yield(Interpreted19Block.java:194)
at org.jruby.runtime.Interpreted19Block.call(Interpreted19Block.java:125)
at org.jruby.runtime.Block.call(Block.java:101)
at org.jruby.RubyProc.call(RubyProc.java:300)
at org.jruby.RubyProc.call19(RubyProc.java:281)
at org.jruby.RubyProc$INVOKER$i$0$0$call19.call(RubyProc$INVOKER$i$0$0$call19.gen)
at org.jruby.internal.runtime.methods.DynamicMethod.call(DynamicMethod.java:210)
at org.jruby.internal.runtime.methods.DynamicMethod.call(DynamicMethod.java:206)
at org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:168)
at org.jruby.ast.CallOneArgNode.interpret(CallOneArgNode.java:57)
at org.jruby.ast.DAsgnNode.interpret(DAsgnNode.java:110)
at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:105)
at org.jruby.ast.BlockNode.interpret(BlockNode.java:71)
at org.jruby.evaluator.ASTInterpreter.INTERPRET_BLOCK(ASTInterpreter.java:112)
at org.jruby.runtime.Interpreted19Block.evalBlockBody(Interpreted19Block.java:206)
at org.jruby.runtime.Interpreted19Block.yield(Interpreted19Block.java:194)
at org.jruby.runtime.Interpreted19Block.call(Interpreted19Block.java:125)
at org.jruby.runtime.Block.call(Block.java:101)
at org.jruby.RubyProc.call(RubyProc.java:300)
at org.jruby.internal.runtime.methods.ProcMethod.call(ProcMethod.java:64)
at org.jruby.internal.runtime.methods.DynamicMethod.call(DynamicMethod.java:210)
at org.jruby.internal.runtime.methods.DynamicMethod.call(DynamicMethod.java:206)
at org.jruby.runtime.callsite.CachingCallSite.call(CachingCallSite.java:168)
...
```

Start with Accessors.java and reproduce the reported field lookup using the configuration and sample event in the issue, where input is a List. Trace the fetch path that parses bidderId, then verify that the resulting diagnostic explains the mismatch instead of exposing NumberFormatException.

Written by the indexing model from the issue text.