Reduce visibility of JsonReply::new

[arminsabouri]

Json replies are carefully crafted to ommit potentially sensative information. Application developers may accidentally stringify implementation errors instead of using the From ReplayableError impl if the ctor is pub.

[coveralls]

Pull Request Test Coverage Report for Build 16777514362

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 85.853%

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Totals
Change from base Build 16750868059:	0.0%
Covered Lines:	7446
Relevant Lines:	8673

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💛 - Coveralls

[nothingmuch]

If you don't mind please also add a unit test for JsonReply::from<ReplyableError::Implementation> not leaking, i think it can be as simple as just ensuring that the message is just "Internal server error", or for bonus cookie points implementing an error whose Display and Debug impls panic, and ensuring that those aren't triggered by JsonReply conversion

[arminsabouri]

If you don't mind please also add a unit test for JsonReply::from<ReplyableError::Implementation> not leaking, i think it can be as simple as just ensuring that the message is just "Internal server error", or for bonus cookie points implementing an error whose Display and Debug impls panic, and ensuring that those aren't triggered by JsonReply conversion

Issue created. Will tackle next