Added 'fail_on_error' config string
Revert "Added 'fail_on_error' config string"

This reverts commit 87406e4d0d2ef2d05097e9aa0b4330e3c3086f2b.
Added "fail_on_error" switch to ignore (and not log) errors, preferring to return fail_on_error=true in the event response - affects elasticsearch query plugin.
Changed logic so default is "true" (normal behaviour)
Setting to "false" switches off exception handling
Formatting, verbose reply message