fix(status): error out when lock not found

cmd/dep/status.go

@@ -261,8 +261,7 @@ func runStatusAll(ctx *dep.Ctx, out outputter, p *dep.Project, sm gps.SourceMana
 	var digestMismatch, hasMissingPkgs bool
 
 	if p.Lock == nil {
-		// TODO if we have no lock file, do...other stuff
-		return digestMismatch, hasMissingPkgs, nil
+		return digestMismatch, hasMissingPkgs, errors.Errorf("no Gopkg.lock found. Run `dep ensure` to generate lock file")
 	}
 
 	// While the network churns on ListVersions() requests, statically analyze

cmd/dep/testdata/harness_tests/status/without_lock/testcase.json

@@ -0,0 +1,6 @@
+{
+  "commands": [
+    ["status"]
+  ],
+  "error-expected": "no Gopkg.lock found. Run `dep ensure` to generate lock file"
+}