Detect git authentication failure (#5420)

Co-authored-by: Michael Oliveira <34169552+Flame442@users.noreply.github.com>

redbot/cogs/downloader/errors.py

@@ -13,6 +13,7 @@ __all__ = [
     "CopyingError",
     "ExistingGitRepo",
     "MissingGitRepo",
+    "AuthenticationError",
     "CloningError",
     "CurrentHashError",
     "HardResetError",
@@ -79,6 +80,15 @@ class MissingGitRepo(DownloaderException):
     pass
 
 
+class AuthenticationError(GitException):
+    """
+    Thrown when git failed to authenticate with
+    the server
+    """
+
+    pass
+
+
 class CloningError(GitException):
     """
     Thrown when git clone returns a non zero exit code.