discovery+routing: split 'routing' package on 'routing' and 'discovery'

In this commit the routing package was divided on two separete one,
this was done because 'routing' package start take too much responsibily
on themself, so with following commit:

Routing pacakge:
Enitites:
* channeldb.ChannelEdge
* channeldb.ChannelPolicy
* channeldb.NodeLightning

Responsibilities:
* send topology notification
* find payment paths
* send payment
* apply topology changes to the graph
* prune graph
* validate that funding point exist and corresponds to given one
* to be the source of topology data

Discovery package:
Entities:
* lnwire.AnnounceSignature
* lnwire.ChannelAnnouncement
* lnwire.NodeAnnouncement
* lnwire.ChannelUpdateAnnouncement

Responsibilities:
* validate announcement signatures
* sync topology with newly connected peers
* handle the premature annoucement
* redirect topology changes to the router susbsystem
* broadcast announcement to the rest of the network
* exchange channel announcement proofs

Before that moment all that was in the 'routing' which is quite big for
one subsystem.

split

routing/pathfind.go

@@ -187,7 +187,8 @@ func newRoute(amtToSend btcutil.Amount, pathEdges []*ChannelHop) (*Route, error)
 		// enough capacity to forward the required amount which
 		// includes the fee dictated at each hop.
 		if nextHop.AmtToForward > nextHop.Channel.Capacity {
-			return nil, ErrInsufficientCapacity
+			return nil, newErrf(ErrInsufficientCapacity, "channel graph has "+
+				"insufficient capacity for the payment")
 		}
 
 		// We don't pay any fees to ourselves on the first-hop channel,
@@ -373,7 +374,8 @@ func findPath(graph *channeldb.ChannelGraph, sourceNode *channeldb.LightningNode
 	// If the target node isn't found in the prev hop map, then a path
 	// doesn't exist, so we terminate in an error.
 	if _, ok := prev[newVertex(target)]; !ok {
-		return nil, ErrNoPathFound
+		return nil, newErrf(ErrNoPathFound, "unable to find a path to "+
+			"destination")
 	}
 
 	// If the potential route if below the max hop limit, then we'll use
@@ -398,7 +400,8 @@ func findPath(graph *channeldb.ChannelGraph, sourceNode *channeldb.LightningNode
 	// hops, then it's invalid.
 	numEdges := len(pathEdges)
 	if numEdges > HopLimit {
-		return nil, ErrMaxHopsExceeded
+		return nil, newErr(ErrMaxHopsExceeded, "potential path has "+
+			"too many hops")
 	}
 
 	// As our traversal of the prev map above walked backwards from the
@@ -515,7 +518,7 @@ func findPaths(graph *channeldb.ChannelGraph, source *channeldb.LightningNode,
 
 			// If we weren't able to find a path, we'll continue to
 			// the next round.
-			if err == ErrNoPathFound {
+			if IsError(err, ErrNoPathFound) {
 				continue
 			} else if err != nil {
 				return nil, err