Add graceful handling of expected exceptions in fuzz_submodule.py

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69350

**`IsADirectoryError`**

Fuzzer provided input data can sometimes produce filenames that look
like directories and raise `IsADirectoryError` exceptions which crash
the fuzzer. This commit catches those cases and returns -1 to instruct
libfuzzer that the inputs are not valuable to add to the corpus.

**`FileExistsError`**

Similar to the above, this is a possible exception case produced by the
fuzzed data and not a bug so its handled the same.

fuzzing/fuzz-targets/fuzz_submodule.py

@@ -67,7 +67,15 @@ def TestOneInput(data):
                 )
                 repo.index.commit(f"Removed submodule {submodule_name}")
 
-        except (ParsingError, GitCommandError, InvalidGitRepositoryError, FileNotFoundError, BrokenPipeError):
+        except (
+            ParsingError,
+            GitCommandError,
+            InvalidGitRepositoryError,
+            FileNotFoundError,
+            FileExistsError,
+            IsADirectoryError,
+            BrokenPipeError,
+        ):
             return -1
         except (ValueError, OSError) as e:
             expected_messages = [