handle format string exceptions separately

gallery_dl/exception.py

@@ -18,6 +18,7 @@ Exception
       |    +-- NotFoundError
       |    +-- HttpError
       +-- NoExtractorError
+      +-- FormatError
       +-- StopExtraction
 """
 
@@ -50,5 +51,9 @@ class NoExtractorError(GalleryDLException):
     """No extractor can handle the given URL"""
 
 
+class FormatError(GalleryDLException):
+    """Error while building output path"""
+
+
 class StopExtraction(GalleryDLException):
     """Extraction should stop"""