more readable errors on ring.Merge
more readable errors on ring.Merge

There are three significant changes here:

- rewrite ring.ErrDifferentSeeds errors
- remove the pre-checking of ranges and instead rewrite
  ring.ErrDifferentRange errors
- don't pruneNicknames() and signal ringUpdated() when there was an
  error

We don't rewrite other errors since our current belief is that only
ErrDifferentSeeds and ErrDifferentRange can arise from user error.

squaremo/weave,dpw/weave,aaam/weave,tomwilkie/weave,bboreham/weave,squaremo/weave,rade/weave,bboreham/weave,paulbellamy/weave,rade/weave,squaremo/weave,awh/weave,aaam/weave,awh/weave,dpw/weave,tomwilkie/weave,awh/weave,rade/weave,dpw/weave,tomwilkie/weave,dpw/weave,weaveworks/weave,paulbellamy/weave,tomwilkie/weave,awh/weave,bboreham/weave,bboreham/weave,n054/weave,tomwilkie/weave,weaveworks/weave,paulbellamy/weave,rade/weave,n054/weave,aaam/weave,awh/weave,paulbellamy/weave,bboreham/weave,squaremo/weave,aaam/weave,weaveworks/weave,n054/weave,n054/weave,rade/weave,dpw/weave,aaam/weave,squaremo/weave,weaveworks/weave,n054/weave