update: get_collection no longer returns Optional

Previously Client.get_collection returned None if the requested collection was
not found. Now it has been updated to raise an exception if the requested
collection does not exist.

Author: ircwaves

pystac_client/client.py

@@ -336,16 +336,17 @@ def _warn_about_fallback(self, *args: str) -> None:
         warnings.warn(FallbackToPystac(), stacklevel=2)
 
     @lru_cache()
-    def get_collection(
-        self, collection_id: str
-    ) -> Optional[Union[Collection, CollectionClient]]:
+    def get_collection(self, collection_id: str) -> Union[Collection, CollectionClient]:
         """Get a single collection from this Catalog/API
 
         Args:
             collection_id: The Collection ID to get
 
         Returns:
             Union[Collection, CollectionClient]: A STAC Collection
+
+        Raises:
+            NotFoundError if collection_id does not exist.
         """
         collection: Union[Collection, CollectionClient]