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: #1939

langchain/document_loaders/url.py

@@ -1,14 +1,17 @@
 """Loader that uses unstructured to load HTML files."""
+import logging
 from typing import List
 
 from langchain.docstore.document import Document
 from langchain.document_loaders.base import BaseLoader
 
+logger = logging.getLogger(__file__)
+
 
 class UnstructuredURLLoader(BaseLoader):
     """Loader that uses unstructured to load HTML files."""
 
-    def __init__(self, urls: List[str]):
+    def __init__(self, urls: List[str], continue_on_failure: bool = True):
         """Initialize with file path."""
         try:
             import unstructured  # noqa:F401
@@ -18,14 +21,21 @@ def __init__(self, urls: List[str]):
                 "`pip install unstructured`"
             )
         self.urls = urls
+        self.continue_on_failure = continue_on_failure
 
     def load(self) -> List[Document]:
         """Load file."""
         from unstructured.partition.html import partition_html
 
         docs: List[Document] = list()
         for url in self.urls:
-            elements = partition_html(url=url)
+            try:
+                elements = partition_html(url=url)
+            except Exception as e:
+                if self.continue_on_failure:
+                    logger.error(f"Error fetching or processing {url}, exeption: {e}")
+                else:
+                    raise e
             text = "\n\n".join([str(el) for el in elements])
             metadata = {"source": url}
             docs.append(Document(page_content=text, metadata=metadata))