feat: Page-level error reporting from PDF backend, introduce PARTIAL_SUCCESS status (#47)

* Put safety-checks for failed parse of pages

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* Introduce page-level error checks

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* Bump to docling-parse 1.1.1

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

* Introduce page-level error checks

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

---------

Signed-off-by: Christoph Auer <cau@zurich.ibm.com>

docling/backend/abstract_backend.py

@@ -7,8 +7,6 @@ from PIL import Image
 
 
 class PdfPageBackend(ABC):
-    def __init__(self, page_obj: Any) -> object:
-        pass
 
     @abstractmethod
     def get_text_in_rect(self, bbox: "BoundingBox") -> str:
@@ -32,6 +30,10 @@ class PdfPageBackend(ABC):
     def get_size(self) -> "PageSize":
         pass
 
+    @abstractmethod
+    def is_valid(self) -> bool:
+        pass
+
     @abstractmethod
     def unload(self):
         pass