Check errors passed through by filepath.Walk (#208)

In several files, the error passed from filepath.Walk to WalkFunc is
not checked. As the info argument to WalkFn is nil in case of an error,
accessing info can cause a runtime panic.
This commit adds checks for errors passed through to WalkFunc.

pkg/git/git.go

@@ -59,6 +59,9 @@ func CloneOrPull(url, repoPath string) (map[string]struct{}, error) {
 	// Need to refresh all vulnerabilities
 	if db.GetVersion() == "" {
 		err = filepath.Walk(repoPath, func(path string, info os.FileInfo, err error) error {
+			if err != nil {
+				return err
+			}
 			if info.IsDir() {
 				return nil
 			}