xenapi: stop agent errors stopping build

We should still report agent errors, but they should not stop a server
from building, because that server still may be ready to use.

Once this change is in place, we can look to selectively add back in
certain errors that fail the build, as we decide what makes sense on a
case by case basis.

Fixes bug 1233548
Change-Id: I32b9e8dbc6e28a045ac8ef159afe1cf74b7ac6fb
Merge "xenapi: stop agent errors stopping build"