Add missing tag name in error message

When creating a new issue via REST API with a non-existing tag name,
the error message generated by mci_tag_set_for_issue() did not include
the tag's name.

This was caused by reusing the $t_tag variable to store the return value
of tag_get_by_name(), so the original data is no longer available when
the exception is thrown.

Same problem in IssueAddCommand::validate() if the user attempting to
create the issue does not have the required privileges to create tags,
this time due to usage of an uninitialized variable.

Fixes 0025996