fix: improve error handling in DocConverter

Enhance the exception handling in the DocConverter class 
to provide more informative error messages. Instead of 
returning None on failure, the code now returns a 
DocumentConverterResult object that includes the error 
message, making it easier to diagnose issues when 
processing DOC files.

src/markitdown/_markitdown.py

@@ -749,8 +749,11 @@ def convert(self, local_path, **kwargs) -> Union[None, DocumentConverterResult]:
             result = self._convert(output_content)
             return result
 
-        except Exception as _:
-            return None
+        except Exception as e:
+            return DocumentConverterResult(
+                title=None,
+                text_content=f"[ERROR] Failed to process DOC file {local_path}: {str(e)}",
+            )
 
 
 class XlsxConverter(HtmlConverter):