Graceful error handling

[chuanqisun]

Currently, there is no error catching and handling for xml parsing error

I think the ideal error should take care of these:

1. The GitHub action should have clear error output.
2. When one source failed, other sources should still be cached correctly.
3. When one source failed, the existing cache of the failing source must be preserved (to avoid data loss). This requires smart cache merging logic.
4. Ideally, the generated UI should alert the user if one or more of the sources have issues. It should draw user's attention but not blocking user from reading existing content. (thank you for the idea @tabokie from Handle enrichment failure gracefully #10)