Change the color of the Synch Mempool trace messages

[Francisco-DAnconia]

On bitcoin mainnet, occasionally there is a failure to sync blocks and/or mempool. This is normal and reflective of the processing of private transactions and/or a full mempool causing a race condition.

Discussed in Issue #277 and Issue #141.

Please change the color of the following strings to yellow from red to indicate the lower severity of the issue:

<SynchMempool> processResults: precache->thread errored out, aborting SynchMempoolTask
<Controller> Failed to synch blocks and/or mempool

Maybe: Keep track of the frequency of this problem, and output these strings in red or another string if this happens often enough to indicate the possibility of a real problem.