Previously, the Conditional class did a simple check when an
AnsibleUndefinedVariable error was raised to see if certain strings were
present. This patch tries to be smarter by evaluating the variable contained
in the error string and compared to the defined/not defined conditionals in
the conditional string.
This also modifies the UndefinedError message from HostVars slightly to
match the format returned jinja2 in general, making it easier to match the
error message in the Conditional code.
Fixes#18514