Update PDF text parser classes. Remove SlateParser and substitute with a PDFMiner based parser.

mayan/apps/ocr/exceptions.py

@@ -8,8 +8,16 @@ class OCRError(Exception):
     pass
 
 
-class UnpaperError(Exception):
+class ParserError(Exception):
     """
-    Raised by unpaper
+    Base exception for file parsers
     """
     pass
+
+
+class NoMIMETypeMatch(ParserError):
+    """
+    There is no parser registered for the specified MIME type
+    """
+    pass
+