add tests for importer failure scenarios

- internal/importers/base/importer_test.go:
  - Check for warning when an invalid project is present whose project root
    cannot be parsed.
  - Check for warning when lock hint cannot be resolved correctly and the
    constraint cannot be applied.
- Integration test for malformed external config(glide.yaml)

cmd/dep/testdata/harness_tests/init/glide/case4/README.md

@@ -0,0 +1 @@
+Ignore glide config if glide.yaml is malformed and cannot be parsed correctly.

cmd/dep/testdata/harness_tests/init/glide/case4/final/Gopkg.toml

@@ -0,0 +1,4 @@
+
+[[constraint]]
+  name = "github.com/sdboyer/deptestdos"
+  version = "2.0.0"

cmd/dep/testdata/harness_tests/init/glide/case4/initial/glide.yaml

@@ -0,0 +1,20 @@
+'package: github.com/golang/notexist
+homepage: http://example.com
+license: MIT
+owners:
+- name: Sam Boyer
+  email: [email protected]
+  homepage: http://sdboyer.io
+ignore:
+- github.com/sdboyer/dep-test
+excludeDirs:
+- samples
+import:
+- package: github.com/sdboyer/deptest # This is a transitive dep and will be ignored
+  repo: https://github.com/sdboyer/deptest.git
+  vcs: git
+  version: v1.0.0
+- package: github.com/sdboyer/deptestdos
+  version: v2.0.0
+testImport:
+- package: github.com/golang/lint

cmd/dep/testdata/harness_tests/init/glide/case4/initial/main.go

@@ -0,0 +1,16 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"fmt"
+
+	"github.com/sdboyer/deptestdos"
+)
+
+func main() {
+	var x deptestdos.Bar
+	fmt.Println(x)
+}

cmd/dep/testdata/harness_tests/init/glide/case4/testcase.json

@@ -0,0 +1,14 @@
+{
+  "commands": [
+    ["init", "-no-examples"]
+  ],
+  "error-expected": "",
+  "gopath-initial": {
+    "github.com/sdboyer/deptest": "3f4c3bea144e112a69bbe5d8d01c1b09a544253f",
+    "github.com/sdboyer/deptestdos": "5c607206be5decd28e6263ffffdcee067266015e"
+  },
+  "vendor-final": [
+    "github.com/sdboyer/deptest",
+    "github.com/sdboyer/deptestdos"
+  ]
+}

internal/importers/base/importer_test.go

@@ -5,6 +5,7 @@
 package base
 
 import (
+	"fmt"
 	"log"
 	"testing"
 
@@ -399,6 +400,32 @@ func TestBaseImporter_ImportProjects(t *testing.T) {
 				},
 			},
 		},
+		"invalid project root": {
+			importertest.TestCase{
+				WantSourceRepo: "",
+				WantWarning:    "Warning: Skipping project. Cannot determine the project root for invalid-project",
+			},
+			[]ImportedPackage{
+				{
+					Name: "invalid-project",
+				},
+			},
+		},
+		"nonexistent project": {
+			importertest.TestCase{
+				WantSourceRepo: "",
+				WantWarning: fmt.Sprintf(
+					"Warning: Unable to apply constraint %q for %s",
+					importertest.V1Tag, importertest.NonexistentPrj,
+				),
+			},
+			[]ImportedPackage{
+				{
+					Name:     importertest.NonexistentPrj,
+					LockHint: importertest.V1Tag,
+				},
+			},
+		},
 	}
 
 	for name, tc := range testcases {

internal/importers/importertest/testdata.go

@@ -61,4 +61,7 @@ const (
 
 	// MultiTaggedPlainTag is a non-semver tag on MultiTaggedRev.
 	MultiTaggedPlainTag = "stable"
+
+	// NonexistentPrj is a dummy project which does not exist on Github.
+	NonexistentPrj = "github.com/nonexistent/project"
 )