Adds stack to error message in postal

postal/service.go

@@ -120,9 +120,10 @@ func (s Service) Resolve(path, id, version, stack string) (Dependency, error) {
 
 	if len(compatibleVersions) == 0 {
 		return Dependency{}, fmt.Errorf(
-			"failed to satisfy %q dependency version constraint %q: no compatible versions. Supported versions are: [%s]",
+			"failed to satisfy %q dependency version constraint %q: no compatible versions on %q stack. Supported versions are: [%s]",
 			id,
 			version,
+			stack,
 			strings.Join(supportedVersions, ", "),
 		)
 	}

postal/service_test.go

@@ -307,7 +307,7 @@ version = "this is super not semver"
 			context("when the entry version constraint cannot be satisfied", func() {
 				it("returns an error with all the supported versions listed", func() {
 					_, err := service.Resolve(path, "some-entry", "9.9.9", "some-stack")
-					Expect(err).To(MatchError(ContainSubstring("failed to satisfy \"some-entry\" dependency version constraint \"9.9.9\": no compatible versions. Supported versions are: [1.2.3, 4.5.6]")))
+					Expect(err).To(MatchError(ContainSubstring("failed to satisfy \"some-entry\" dependency version constraint \"9.9.9\": no compatible versions on \"some-stack\" stack. Supported versions are: [1.2.3, 4.5.6]")))
 				})
 			})
 		})