New X509D2iError to improve error checking for X509_ext_get_d2i()

[HolyShitMan]

First try to fix #2226:
New X509D2iError which represents the errors that can occur after calling X509_ext_get_d2i()

pub enum X509D2iError {
    InternalOpenSSLError(ErrorStack),
    ExtensionNotFoundError,
    ExtensionAmbiguousError,
}

All corresponding X509_ext_get_d2i() calls are adapted accordingly to not return Option<...>, but a Result. ExtensionNotFoundError is handled as None before.

If you got other ideas or expectations let me know.

[HolyShitMan]

Hi @botovq,
I have a couple of questions regarding this PR:

• Do these changes align with your vision for resolving issue Improve error checking for X509_ext_get_d2i() #2226?
• Who will be responsible for reviewing these changes?

Thank you!

[botovq]

The approach looks sensible to me. I do not know when and how this crate is willing to do breaking changes to its public API. Maybe the less brutal way to do it would be to deprecate the existing API and point at new variants that return Result<_, X509D2iError>. It might even be possible to make the existing API wrappers of the new API that map the X509D2iError to None (this has upsides and downsides).

That's @sfackler and @alex's call, though.