pdnsutil add-zone-key: more checks

[miodvallat]

Short description

Old issues #1304 and #2855 describe situations where pdnsutil add-zone-key failed yet did not report failure.

I believe the current state of the code will correctly report failure, but it does not hurt to be more careful in pdnsutil. The changes in this PR achieve two things:

• pdnsutil add-zone-key will exit with non-zero status if attempting to get the key ID from the backend failed (for we can not be sure a key had been added in this case).
• if the backend is able to return a key ID, an extra request to fetch the key with this ID will be performed, to confirm the key has correctly be created; if this fetch fails, it will report the error and exit with a non-zero status code.

The first two commits are purely janitorial cleanup; the important change is found in the third commit. Commit-by-commit review recommended.

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)