Improve error handling during var resolution.

pkg/expansion/expand.go

@@ -44,7 +44,6 @@ func MappingFuncFor(context ...map[string]string) func(string) string {
 				return val
 			}
 		}
-
 		return syntaxWrap(input)
 	}
 }

pkg/expansion/expand_test.go

@@ -46,9 +46,7 @@ func TestMapReference(t *testing.T) {
 		"BLU": "$(ZOO)-2",
 	}
 
-	serviceEnv := map[string]string{}
-
-	mapping := MappingFuncFor(declaredEnv, serviceEnv)
+	mapping := MappingFuncFor(declaredEnv)
 
 	for _, env := range envs {
 		declaredEnv[env.Name] = Expand(env.Value, mapping)