Uniformise `%ex` pattern and `exception` JTL resolver

[ppkarwasz]

Description

Currently there is a difference in the way exceptions are formatted by the %ex pattern converter and the following JSON Template Layout snippet:

{
  "$resolver": "exception",
  "field": "stackTrace",
  "stackTrace": {
    "stringified": true
  }
}

• JTL simply uses Throwable.printStackTrace() and the first line of the output is obtained by calling Object.toString().
• Pattern Layout uses getClass().getName() + ": " + getLocalisedMessage().

Those lines agree, unless an exception overrides Object#toString().