Fixes #36914 - skip missing hosts during applicability generate

[ianballou]

What are the changes introduced in this pull request?

Adds error handling to the block in BulkGenerate that calculates applicability for all hosts.

The issue here is that we cannot use resource locking because it's a list of hosts. So, we can't stop hosts from being deleted during applicability calculation.

We thought we handled this before with am existence check, but turns out some folks still hit the issue because the host is deleted part way through calculate_and_import_applicability.

NoMethodError and all PG errors are handled here to cover our bases but not be too broad. Plus the errors are raised back to the logs so debugging shouldn't be too much harder.

Also makes bulk generate skip itself on an error so the task doesn't get paused.

Considerations taken when implementing this change?

I don't see any reason why applicability generation needs to result in a paused task. I don't see it causing a data integrity issue.

What are the testing steps for this pull request?

1. Edit the code to error out somewhere in calculate_and_import_applicability
2. Trigger applicability generation for a host, ideally the "natural" way via subscription-manager
3. See that the error is handled and that it pops up in the logs.

[sjha4]

Code looks good. Task now skips and ends upon failure and is able to log errors when error introduced in calculate_and_import_applicability method instead of pausing.