crypto/x509: CreateCertificate should return a meaningful error instead of panic when passed an unknown ExtKeyUsage

[rolandshoemaker]

x509.CreateCertificate panics if passed a template containing an unknown ExtKeyUsage, example. Probably it'd make more sense to return an error along the lines of Unknown extended key usage.

[FiloSottile]

(In general, I'd like all crypto functions with an error return value to stop ever returning a panic for any reason at all.)

[tpaschalis]

I'll give this a shot tomorrow if there's no objection. I see three cases where a panic is called in the crypto/x509 package, we could possibly eliminate all of them.

[gopherbot]

Change https://golang.org/cl/252557 mentions this issue: crypto/x509: return error when passing unknown ExtKeyUsage