Fixed Issue 175: Command does not load dotenv files if some do not exist

fixtures/valid.env

@@ -0,0 +1 @@
+FOO=BAR

godotenv.go

@@ -137,7 +137,10 @@ func Unmarshal(str string) (envMap map[string]string, err error) {
 // If you want more fine grained control over your command it's recommended
 // that you use `Load()` or `Read()` and the `os/exec` package yourself.
 func Exec(filenames []string, cmd string, cmdArgs []string) error {
-	Load(filenames...)
+	err := Load(filenames...)
+	if err != nil {
+		return err
+	}
 
 	command := exec.Command(cmd, cmdArgs...)
 	command.Stdin = os.Stdin

godotenv_test.go

@@ -2,10 +2,12 @@ package godotenv
 
 import (
 	"bytes"
+	"errors"
 	"fmt"
 	"os"
 	"reflect"
 	"strings"
+	"syscall"
 	"testing"
 )
 
@@ -472,3 +474,11 @@ func TestRoundtrip(t *testing.T) {
 
 	}
 }
+
+func TestExecWhenFileNotFound(t *testing.T) {
+	fileNames := []string{"not-existed-file-name.env"}
+	err := Exec(fileNames, "", []string{})
+	if 	!errors.Is(err, syscall.ERROR_FILE_NOT_FOUND) {
+		t.Error("Expected: return error when system cannot find the file specified")
+	}
+}