logstash illegal_argument_exception reasons can be unclear

- Version:
logstash 2.2.4
- Operating System:
centos7 64
- Config File (if you have sensitive info, please remove it):
`
filter {
xml {
source => "message"
target => "testsuites"
store_xml => true
}
}
`
- Sample Data:

```

```
- Steps to Reproduce:
create an index with a `testsuite` string field.
send that xml file to logstash (i'm using http input plugin)

the testsuite field collides because it's already a string and can't be a nested field.
logstash gives less than helpful error (logstash.log):

```
"status"=>400, "error"=>{"type"=>"mapper_parsing_exception", "reason"=>"failed to parse [data]", "caused_by"=>{"type"=>"illegal_argument_exception", "reason"=>"unknown property [testsuite]"}}}}, :level=>:warn}
```