Capitalization format makes erroring code hard to search for in codebase

For example: I was investigating a `HeaderVerificationFailure` error. I searched for `HeaderVerificationFailure` to see what code produced it, and came up empty-handed.

With further investigation, I was able to figure out where it came from, and I found that the code producing the error uses the syntax `Error::header_verification_failure(e.to_string()))` instead of something containing `HeaderVerificationFailure`.

IMO, this places an unnecessary overhead on developers debugging code using this library.