refactor: Remove pymupdf coupling in extraction

The library is only needed for pdf extraction which is taken care of
in its own extractor plugin. In the overall extraction routine we do not
need any knowledge of the existence of pymupdf.

papis_extract/extractors/__init__.py

@@ -16,3 +16,13 @@ if find_spec("bs4") and find_spec("magic"):
     all_extractors["pocketbook"] = PocketBookExtractor()
 else:
     logger.debug("pocketbook extractor not activated.")
+
+
+class ExtractionError(Exception):
+    """Raised for exceptions during extraction.
+
+    Something went wrong during the extraction process in the extractor
+    run routine itself.
+    """
+
+    pass