auth LUA createForward6: on parse problem, return :: instead of SERVFAIL

[Habbie]

Short description

this mirrors how we handle the v4 case

closes #14629

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 10737967328

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 14 (0.0%) changed or added relevant lines in 1 file are covered.
• 34 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.2%) to 64.789%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/lua-record.cc	0	14	0.0%

Files with Coverage Reduction	New Missed Lines	%
ext/json11/json11.cpp	1	64.63%
pdns/recursordist/recursor_cache.cc	1	86.42%
pdns/backends/gsql/gsqlbackend.hh	2	97.71%
pdns/recursordist/syncres.cc	2	79.55%
pdns/rcpgenerator.cc	2	90.14%
pdns/opensslsigners.cc	3	61.41%
pdns/remote_logger.cc	3	54.71%
pdns/misc.cc	3	63.49%
pdns/signingpipe.cc	5	84.68%
pdns/recursordist/test-syncres_cc1.cc	6	89.87%

Totals
Change from base Build 10717986498:	0.2%
Covered Lines:	124847
Relevant Lines:	162107

---

💛 - Coveralls