server/asset/btc: recognize errNoFeeRate

[chappjc]

The server logs spam like this every epoch (with two DOGE markets):

2022-06-08 04:35:30.000 [TRC] ASSET[doge]: Estimatefee error for doge: fee could not be estimated                                                                                                    
2022-06-08 04:35:30.001 [DBG] ASSET[doge]: Fee estimate unavailable for doge. Using median fee.                                                                                                      
2022-06-08 04:35:30.001 [TRC] ASSET[doge]: Using cached doge median fee rate                                                                                                                         2022-06-08 04:35:30.001 [TRC] ASSET[doge]: Estimatefee error for doge: fee could not be estimated                                                                                                    
2022-06-08 04:35:30.001 [DBG] ASSET[doge]: Fee estimate unavailable for doge. Using median fee.                                                                                                      
2022-06-08 04:35:30.001 [TRC] ASSET[doge]: Using cached doge median fee rate 

This PR cools off the logs a bit by introducing errNoFeeRate to go along with errNoCompetition. The errNoFeeRate is just used by the estimate fee methods when there's no error from the RPC, but the rate is zero. This leads to median fee estimation based on past blocks (or a cached rate) and is generally not an error condition.

This also removes btc.name from many of the log statements in btc.Backend because the logger's prefix already contains the asset, e.g. [DBG] ASSET[doge]: Blocks are too empty to calculate median fees. Using no-competition rate (4000).