UnstructuredURLLoader: allow url failures, keep processing (#1954)
By default, UnstructuredURLLoader now continues processing remaining
`urls` if encountering an error for a particular url.
If failure of the entire loader is desired as was previously the case,
use `continue_on_failure=False`.
E.g., this fails splendidly, courtesy of the 2nd url:
```
from langchain.document_loaders import UnstructuredURLLoader
urls = [
"https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-february-8-2023",
"https://doesnotexistithinkprobablynotverynotlikely.io",
"https://www.understandingwar.org/backgrounder/russian-offensive-campaign-assessment-february-9-2023",
]
loader = UnstructuredURLLoader(urls=urls, continue_on_failure=False)
data = loader.load()
```
Issue: https://github.com/hwchase17/langchain/issues/1939