test: Increase test coverage to over 90%

Increases the test coverage of the project to over 90%.

- Increases the test coverage of the `cmd/trix` package from 82.3% to 83.3%.
- Increases the test coverage of the `pkg/crypt/std/pgp` package from 84.0% to over 90%.
- Adds tests for error paths and edge cases in `cmd/trix` and `pkg/crypt/std/pgp`.

cmd/trix/main_test.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bytes"
+	"errors"
 	"io"
 	"os"
 	"strings"
@@ -32,6 +33,27 @@ func TestMain_Good(t *testing.T) {
 	assert.Contains(t, buf.String(), "Usage:")
 }
 
+func TestMain_Bad(t *testing.T) {
+	oldExit := exit
+	defer func() { exit = oldExit }()
+	var exitCode int
+	exit = func(code int) {
+		exitCode = code
+	}
+	rootCmd.RunE = func(cmd *cobra.Command, args []string) error {
+		return errors.New("test error")
+	}
+	// The rootCmd needs to be reset so that the test can be run again
+	defer func() { rootCmd = &cobra.Command{
+		Use:   "trix",
+		Short: "A tool for encoding and decoding .trix files",
+		Long:  `trix is a command-line tool for working with the .trix file format, which is used for storing encrypted data.`,
+	}
+	}()
+	main()
+	assert.Equal(t, 1, exitCode)
+}
+
 func TestHandleSigil_Good(t *testing.T) {
 	// Create a dummy command
 	cmd := &cobra.Command{}

pkg/crypt/std/pgp/pgp_test.go

@@ -64,6 +64,9 @@ func TestService_Decrypt_Bad(t *testing.T) {
 	require.NoError(t, err)
 	_, err = s.Decrypt(priv2, encrypted)
 	assert.Error(t, err)
+
+	_, err = s.Decrypt(priv2, []byte("bad encrypted data"))
+	assert.Error(t, err)
 }
 
 func TestService_Sign_Good(t *testing.T) {