Add code to fail with explicit error messages when required attribute…

[kjpye]

…s are missing for new

connections and statement handles.

These are probably never used by end users, but explicit messages never hurt.

[JJ]

There seems to be several errors with the latest 2018.06 version of Perl. Can you please check? https://travis-ci.org/perl6/DBIish/jobs/414423720

[salortiz]

@JJ

This request to add logic that, at first, was implemented by the original required trait.
So, in my local copy I simply reverted your commit 399447b. All test passed:

[sog@monica DBIish]$ prove -e 'perl6 -I./lib' t/*
t/01-Basic.t .............. ok
…
All tests successful.

Test Summary Report
-------------------
t/03-lib-util.t         (Wstat: 0 Tests: 5 Failed: 0)
  TODO passed:   4-5
Files=22, Tests=788,  7 wallclock secs ( 0.07 usr  0.02 sys +  9.53 cusr  0.70 csys = 10.32 CPU)
Result: PASS
[sog@monica DBIish]$ perl6 -v
This is Rakudo version 2018.06-404-g0b3fc077e built on MoarVM version 2018.06-399-g4fc267b60
implementing Perl 6.c.

Seems that is required is now working.

Regards

[JJ]

I am a bit lost here, but the errors in CI do not seem to have anything to do with the previous PR (which passed).

[salortiz]

Commit 33ada3e properly address the intent of this PR and the original issue #123