Handle regex parsing errors in Gsub and Grok Processors

[talevy]

Currently, both Gsub and Grok parse regex strings during
Pipeline creation. Thrown parsing exceptions were leaking out, this
commit wraps those exceptions in ElasticsearchParseExceptions.

Gsub Example

{
  "pipeline": {
    "processors": [
      {
        "gsub": {
          "tag": "processor_1",
          "field": "message",
          "pattern": "[",
          "replacement": ""
        }
      }
    ]
  }
}

expected error:

{
   "error": {
      "root_cause": [
         {
            "type": "parse_exception",
            "reason": "[pattern] Invalid regex pattern. Unclosed character class near index 0\n[\n^",
            "header": {
               "processor_type": "gsub",
               "processor_tag": "processor_1",
               "property_name": "pattern"
            }
         }
      ],
      "type": "parse_exception",
      "reason": "[pattern] Invalid regex pattern. Unclosed character class near index 0\n[\n^",
      "header": {
         "processor_type": "gsub",
         "processor_tag": "processor_1",
         "property_name": "pattern"
      }
   },
   "status": 400
}

Grok Example

{
  "pipeline": {
    "processors": [
      {
        "grok": {
          "tag": "processor_1",
          "field": "message",
          "pattern": "["
        }
      }
    ]
  }
}

and expected error:

{
   "error": {
      "root_cause": [
         {
            "type": "parse_exception",
            "reason": "[pattern] Invalid regex pattern. premature end of char-class",
            "header": {
               "processor_type": "grok",
               "processor_tag": "processor_1",
               "property_name": "pattern"
            }
         }
      ],
      "type": "parse_exception",
      "reason": "[pattern] Invalid regex pattern. premature end of char-class",
      "header": {
         "processor_type": "grok",
         "processor_tag": "processor_1",
         "property_name": "pattern"
      }
   },
   "status": 400
}

[martijnvg]

LGTM