[dl:http] fix potential FileExistsError when '.part' file moved (#5385)

when the downloader detects a '.part' file and attempts to resume its
download, a FileExistsError was raised while trying to create an assumed
missing directory path when said '.part' file was (re)moved during the
time it was detected and the downloader attempted to open it in 'a+b' mode.

gallery_dl/downloader/http.py

@@ -308,6 +308,9 @@ class HttpDownloader(DownloaderBase):
             # download content
             self.downloading = True
             with pathfmt.open(mode) as fp:
+                if fp is None:
+                    # '.part' file no longer exists
+                    break
                 if file_header:
                     fp.write(file_header)
                     offset += len(file_header)

gallery_dl/path.py

@@ -171,6 +171,9 @@ class PathFormat():
         try:
             return open(self.temppath, mode)
         except FileNotFoundError:
+            if "r" in mode:
+                # '.part' file no longer exists
+                return util.NullContext()
             os.makedirs(self.realdirectory)
             return open(self.temppath, mode)