expose dependency mapping error to caller

gh-220

postal/service.go

@@ -142,8 +142,9 @@ func (s Service) Resolve(path, id, version, stack string) (Dependency, error) {
 func (s Service) Deliver(dependency Dependency, cnbPath, layerPath, platformPath string) error {
 	dependencyMappingURI, err := s.mappingResolver.FindDependencyMapping(dependency.SHA256, filepath.Join(platformPath, "bindings"))
 	if err != nil {
-		return fmt.Errorf("failure checking out the bindings")
+		return fmt.Errorf("failure checking for dependency mappings: %s", err)
 	}
+
 	if dependencyMappingURI != "" {
 		dependency.URI = dependencyMappingURI
 	}

postal/service_test.go

@@ -702,6 +702,16 @@ version = "this is super not semver"
 				})
 			})
 		})
+		context("when dependency mapping resolver fails", func() {
+			it.Before(func() {
+				mappingResolver.FindDependencyMappingCall.Returns.Error = fmt.Errorf("some dependency mapping error")
+			})
+			it("fails to find dependency mappings", func() {
+				err := deliver()
+
+				Expect(err).To(MatchError(ContainSubstring("some dependency mapping error")))
+			})
+		})
 	})
 
 	context("Install", func() {