make Failure pub(readonly)

[tonyfettes]

This PR makes Failure has visibility pub(readonly).

[peter-jerry-ye-code-review]

‼️ This code review is generated by a bot. Please verify the content before trusting it.

From the provided git diff, here are potential issues I've noticed:

1. Inconsistency in visibility modifiers: The Failure type was changed from pub(all) to pub, which might affect code that was relying on the previous visibility scope. This could cause compilation errors in code that was using the pub(all) access level.

2. Direct constructor usage vs. new method: In the test case, the code is updated to use Failure::new() but there might be other places in the codebase still using the direct constructor Failure(). These instances should be updated to use the new constructor method for consistency.

3. Missing documentation: The newly added Failure::new() constructor method lacks documentation comments (///), which is inconsistent with the documentation style visible in other parts of the code. This could make the API harder to understand for other developers.

These are potential issues that might need attention, though their severity depends on the specific context and requirements of your project.

[coveralls]

Pull Request Test Coverage Report for Build 4924

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.01%) to 82.926%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtin/failure.mbt	0	1	0.0%

Totals
Change from base Build 4921:	-0.01%
Covered Lines:	5051
Relevant Lines:	6091

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💛 - Coveralls

[bobzhang]

what's the motivation here?

[tonyfettes]

I'm not sure on this change as well. My motivations for this change is, providing a smart constructor to error type allow us to:

1. Change the inner implementation without breaking user code,
2. We can deprecate it.

However here are my cons for this change:

1. This will break user code anyway,
2. Poor performance (but it's less important since we're through error).