Add ext/random Exception hierarchy

[TimWolla]

This complements #9211.

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The hierarchy is roughly based on @Danack's recommendations in this comment: #9071 (comment).

Choice of base Exception

RandomError and RandomException both represent the base Exceptions. I considered using an interface, but thus would make it harder to not catch Error / RandomError:

You can easily do catch (\Random\RandomError|\Random\RandomException) if you want to catch both Errors and Exceptions for whatever reason. But doing catch ((\Random\RandomExceptionInterface & \Random\RandomException)) is not legal if you want to catch just Exceptions, but not Errors.

BrokenRandomEngineError

While you should not catch an Error, I found it useful to have a meaningful class name here. BrokenRandomEngineError is used for:

• A user engine that returns an empty string (which is equivalent to 0 bits of randomness and thus disallowed).
• An engine that fails to generate acceptable randomness for use with range() (e.g. getInt()) in 50 attempts. The chance of rejection for a uniform bytestring is less that 50%. Thus the chance of failure for an unbiased engine is less than 0.5⁵⁰ (~ 8.8×10^-16). If an engine fails that then it must be heavily biased.

RandomException

This is used directly for:

• Failure of the CSPRNG in random_bytes() and random_int() (which should not happen on modern OSes).
• Failure to seed Mt19937, Xoshiro256StarStar, PcgOneseq128XslRr64 from the CSPRNG when a null seed is used to improve the error reporting as per Improve error message of an engine fails to seed from the CSPRNG #9160. Thus the failure mode is identical to the one above (none of modern systems).

[zeriyoshi]

This is so clean and beautiful!

[Danack]

Prediction, the name "RandomException" will be used by multiple people as a point of humour in talks about the new and improved random capabilities of PHP.

So, all looks good to me.

[Girgias]

LGTM

[cmb69]

Thank you!