Display underlying error when verifying API key

[jhchabran]

Which problem is this PR solving?

• The error message Unable to create trace URL: unable to verify API key could be clearer. This PR prints the underlying error. Here how I came up to this:
  • I had misread the BUILDEVENT_APIKEY in the docs and actually used BUILDEVENT_API_KEY
  • I thus wrongly assumed there was some issue elsewhere because even re-reading the docs, the missing _ is hard to see (that's on me!)
  • I applied the patch in this PR to actually see what was the issue.
  • It showed me a message about the key being blank, so it finally clicked and I saw the missing _.

At this point, I thought I'd rather submit a PR just in case you find it appropriate :)

This (probably?) addresses #92.

Short description of the changes

• It just prints the underlying error from libhoney when it cannot verify the API key.

[MikeGoldsmith]

Looks good, thank you for your contribution @jhchabran 👍🏻