gaia/app: failure to retrieve the userHome directory should crash loudly instead of just fmt.Println and inserting content into current directory, or at least fallback to using os.TempDir

[odeke-em]

Summary of Bug

This code is suspect and it os.UserHomeDir fails, it just incorrectly prints to screen and then uses the empty string to create a filepath which will use the current working directory
in

gaia/app/app.go

Lines 85 to 92 in 842f231

	func init() {
	userHomeDir, err := os.UserHomeDir()
	if err != nil {
	stdlog.Println("Failed to get home dir %2", err)
	}
	DefaultNodeHome = filepath.Join(userHomeDir, ".gaia")
	}

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