Add status feedback on memo mismatch

- Adds feedback to status when there's a lock memo mismatch. For
mismatch with missing packages, it lists the packages and provides
instructions to fix the issue. For memo mismatch due to modified
manifest, it instructs the user about how to generate a new lock.

cmd/dep/status.go

@@ -214,11 +214,24 @@ func (cmd *statusCommand) Run(ctx *dep.Ctx, args []string) error {
 		}
 	}
 
-	if err := runStatusAll(ctx.Loggers, out, p, sm); err != nil {
+	memoMismatch, hasMissingPkgs, err := runStatusAll(ctx.Loggers, out, p, sm)
+	if err != nil {
 		return err
 	}
 
-	ctx.Loggers.Out.Print(buf.String())
+	if memoMismatch {
+		if hasMissingPkgs {
+			ctx.Loggers.Err.Println("Lock memo mismatch due to the following missing packages:\n")
+			ctx.Loggers.Out.Print(buf.String())
+			ctx.Loggers.Err.Println("\nThis happens when a new import is added. Run `dep ensure` to install the missing packages.")
+		} else {
+			ctx.Loggers.Err.Printf("Lock memo mismatch. This happens when Gopkg.toml is modified.\n" +
+				"Run `dep ensure` to regenerate the memo.")
+		}
+	} else {
+		ctx.Loggers.Out.Print(buf.String())
+	}
+
 	return nil
 }
 
@@ -239,17 +252,19 @@ type MissingStatus struct {
 	MissingPackages []string
 }
 
-func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.SourceManager) error {
+func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.SourceManager) (bool, bool, error) {
+	var memoMismatch, hasMissingPkgs bool
+
 	if p.Lock == nil {
 		// TODO if we have no lock file, do...other stuff
-		return nil
+		return memoMismatch, hasMissingPkgs, nil
 	}
 
 	// While the network churns on ListVersions() requests, statically analyze
 	// code from the current project.
 	ptree, err := pkgtree.ListPackages(p.AbsRoot, string(p.ImportRoot))
 	if err != nil {
-		return errors.Errorf("analysis of local packages failed: %v", err)
+		return memoMismatch, hasMissingPkgs, errors.Errorf("analysis of local packages failed: %v", err)
 	}
 
 	// Set up a solver in order to check the InputHash.
@@ -266,7 +281,7 @@ func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.So
 
 	s, err := gps.Prepare(params, sm)
 	if err != nil {
-		return errors.Errorf("could not set up solver for input hashing: %s", err)
+		return memoMismatch, hasMissingPkgs, errors.Errorf("could not set up solver for input hashing: %s", err)
 	}
 
 	cm := collectConstraints(ptree, p, sm)
@@ -297,7 +312,7 @@ func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.So
 				ptr, err := sm.ListPackages(proj.Ident(), proj.Version())
 
 				if err != nil {
-					return fmt.Errorf("analysis of %s package failed: %v", proj.Ident().ProjectRoot, err)
+					return memoMismatch, hasMissingPkgs, fmt.Errorf("analysis of %s package failed: %v", proj.Ident().ProjectRoot, err)
 				}
 
 				prm, _ := ptr.ToReachMap(true, false, false, nil)
@@ -359,7 +374,7 @@ func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.So
 		}
 		out.BasicFooter()
 
-		return nil
+		return memoMismatch, hasMissingPkgs, nil
 	}
 
 	// Hash digest mismatch may indicate that some deps are no longer
@@ -368,6 +383,7 @@ func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.So
 	//
 	// It's possible for digests to not match, but still have a correct
 	// lock.
+	memoMismatch = true
 	rm, _ := ptree.ToReachMap(true, true, false, nil)
 
 	external := rm.FlattenOmitStdLib()
@@ -400,7 +416,7 @@ func runStatusAll(loggers *dep.Loggers, out outputter, p *dep.Project, sm gps.So
 			loggers.Err.Printf("\t%s: %s\n", fail.ex, fail.err.Error())
 		}
 
-		return errors.New("address issues with undeducable import paths to get more status information.")
+		return memoMismatch, hasMissingPkgs, errors.New("address issues with undeducable import paths to get more status information.")
 	}
 
 	out.MissingHeader()
@@ -415,11 +431,12 @@ outer:
 			}
 		}
 
+		hasMissingPkgs = true
 		out.MissingLine(&MissingStatus{ProjectRoot: string(root), MissingPackages: pkgs})
 	}
 	out.MissingFooter()
 
-	return nil
+	return memoMismatch, hasMissingPkgs, nil
 }
 
 func formatVersion(v gps.Version) string {