Specify what is missing if tag assignment failed

[yrudman]

ISSUE:
if tag category or tag name misspelled when executing API call to assign tag than return message did not indicate what went wrong

Example:
curl --user admin:smartvm -i -X POST -d '{"action":"assign","resources":[{"category":"team","name":"it_pnp"}]}' http://localhost:3000/api/vms/54/tags
BEFORE:
{"results":[{"success":false,"message":"Assigning Tag: category:'team' name:'it_pnp'", ...}
AFTER:
{"results":[{"success":false,"message":"Tag category 'team' not found in region 0", ...}

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1792106

[miq-bot]

Checked commits yrudman/manageiq@3a56e0e~...3e8b342 with ruby 2.5.7, rubocop 0.69.0, haml-lint 0.28.0, and yamllint
2 files checked, 0 offenses detected
Everything looks fine. ⭐

[yrudman]

@miq-bot add-label ivanchuk/yes, jansa/yes

[simaishi]

Removing /yes labels as this was reverted in #20214