errno_from_exception: require int in args fallback

[HrachShah]

Summary

errno_from_exception returns the exception's errno attribute when present, otherwise it falls back to args[0]. Every internal caller compares the result against an int errno constant (e.g. errno_from_exception(e) == errno.EBADF), so the args fallback has to return an int.

The pre-fix code returned args[0] as-is, which meant a str message — the shape used by most third-party OSError subclasses, by OSError subclasses that only set a human-readable message, and by any ValueError / RuntimeError raised with a string — flowed into the int-comparison call sites and silently turned every comparison into False. The real error class (and the right recovery branch) was hidden.

Fix

Pin the fallback to return args[0] only when it is an int, otherwise None — which matches the call sites' expectations and surfaces the "unknown errno" case the same way as the empty-args path.

Tests

Six new tests in tornado/test/util_test.py:ErrnoFromExceptionTest:

• test_errno_attr_wins_over_args
• test_args0_int_returned_when_no_errno_attr
• test_empty_args_returns_none (the function used to raise TypeError here)
• test_non_int_args0_returns_none
• test_non_int_args0_with_int_args1_returns_none
• test_message_only_args_returns_none

All 6 pass. The 6 new tests fail on the pre-fix code (with TypeError for the empty-args case and the wrong return value for the rest) and pass with the fix.