Revise errors for CertificateSigningRequestParams::from_der

[dwhjames]

• Introduce specific error for CSR signature verification
• Make error name more specific for unsupported CSR extensions

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Following up from

• Expand rustdoc for CertificateSigningRequestParams::from_der #386

[dwhjames]

I realize this is a breaking change (hence the semver warning).

Happy to roll this part back, or move to a separate PR.

[djc]

Suggest you leave the old names in place, marked with #[deprecated] with appropriate since and note, and add the new variants next to them (in the same commit).

[dwhjames]

Rolled this back in 5fd069c and will split off.

[djc]

It would be nice to better understand the motivation/use case here.

[dwhjames]

It would be nice to better understand the motivation/use case here.

@djc, I’m assuming the original introduction of Error::RingUnspecified is for

rcgen/rcgen/src/key_pair.rs

Lines 651 to 656 in 453bcb5

	#[cfg(feature = "crypto")]
	impl<T> ExternalError<T> for Result<T, ring_error::Unspecified> {
	fn _err(self) -> Result<T, Error> {
	self.map_err(|_| Error::RingUnspecified)
	}
	}

where a catch-all error case of the underlying crypto library is lifted into the rcgen error enumeration.

And later it was re-used in this case

• 116df85#diff-b1a35a68f14e696205874893c07fd24fdb88882b47c23cc0e0c80a30c7d53759R508

It would be clearer in code that uses this library to have error variant names that map in a self-evident way to the expected error conditions expected in parsing and verifying a CSR.

Some, but not all, existing error variants include contextual hints in the name (…InCsr or …CertificationRequest are existing examples).

[djc]

It would be nice to better understand the motivation/use case here.

You still haven't really answered this question. What problem did you run into that you are trying to solve?