A refusal for want of a provider names the modules another node cannot run in this composition, and a refusal naming no placed module is answered at once (issue 1126, probe #532)

ace and g14 answered 'nothing in this mesh provides artifact-store … assign
distribution to a node' while novox runs it: a module another node cannot run
in this composition (catalogue Resolution.Unhostable) offers nothing, silently.
The refusal now names each such module and what it lacks, and each node left
out. And the culprit search composed the whole mesh once per placed module for a
refusal no stub changes (ace's answer took 7m46s): only the modules the refusal
names are tried, at most 8.

go/compose/network.go

@@ -24,6 +24,9 @@ type composer struct {
 	in   Input
 	said []string
 	seen map[string]bool
+	// unhostable is, from the last look at the rest of the mesh, each module assigned to another node that this
+	// composition finds that node cannot run: what it would have offered is missing here (issue 1126, probe #532).
+	unhostable []string
 }
 
 func newComposer(in Input) *composer { return &composer{in: in, seen: map[string]bool{}} }