Handle not-enough-votes response in a more friendly way

[pgerzani]

Currently, when getblocktemplate returns the not-enough-votes response and is a 500 error, the miner log shows just that it was a 500 error:

[2020-09-23 02:08:43] HTTP request failed: The requested URL returned error: 500 Internal Server Error
[2020-09-23 02:08:43] HTTP request failed: The requested URL returned error: 500 Internal Server Error

It would be best if instead it said:

[2020-09-23 02:08:43] Waiting for new work -- Not enough votes for currently block

or something comparable. This likely will change when we switch it to continue mining on the previous block as well, but I figured it would be a quick fix regardless.

[sebastianrusu]

Resolved in #7.