improve 'extractor.request'

- add 'fatal' argument
- improve internal logic and flow
- raise known exception on error
- update exception hierarchy

gallery_dl/exception.py

@@ -6,22 +6,49 @@
 # it under the terms of the GNU General Public License version 2 as
 # published by the Free Software Foundation.
 
+"""Exception classes used by gallery-dl
 
-class NoExtractorError(Exception):
-    """No extractor can handle the given URL"""
+Class Hierarchy:
+
+Exception
+ +-- GalleryDLException
+      +-- ExtractionError
+      |    +-- AuthenticationError
+      |    +-- AuthorizationError
+      |    +-- NotFoundError
+      |    +-- HttpError
+      +-- NoExtractorError
+      +-- StopExtraction
+"""
 
 
-class AuthenticationError(Exception):
+class GalleryDLException(Exception):
+    """Base class for GalleryDL exceptions"""
+
+
+class ExtractionError(GalleryDLException):
+    """Base class for exceptions during information extraction"""
+
+
+class AuthenticationError(ExtractionError):
     """Invalid or missing login information"""
 
 
-class AuthorizationError(Exception):
+class AuthorizationError(ExtractionError):
     """Insufficient privileges to access a resource"""
 
 
-class NotFoundError(Exception):
+class NotFoundError(ExtractionError):
     """Requested resource (gallery/image) does not exist"""
 
 
-class StopExtraction(Exception):
+class HttpError(ExtractionError):
+    """HTTP request during extraction failed"""
+
+
+class NoExtractorError(GalleryDLException):
+    """No extractor can handle the given URL"""
+
+
+class StopExtraction(GalleryDLException):
     """Extraction should stop"""