chore: tidy up pkgs (#636)

for readability and towards potentially making helmfile usable as a go library

pkg/helmexec/exit_error.go

@@ -0,0 +1,36 @@
+package helmexec
+
+import (
+	"fmt"
+	"path/filepath"
+	"strings"
+)
+
+func newExitError(helmCmdPath string, exitStatus int, errorMessage string) ExitError {
+	return ExitError{
+		msg:        fmt.Sprintf("%s exited with status %d:\n%s", filepath.Base(helmCmdPath), exitStatus, indent(strings.TrimSpace(errorMessage))),
+		exitStatus: exitStatus,
+	}
+}
+
+func indent(text string) string {
+	lines := strings.Split(text, "\n")
+	for i := range lines {
+		lines[i] = "  " + lines[i]
+	}
+	return strings.Join(lines, "\n")
+}
+
+// ExitError is created whenever your shell command exits with a non-zero exit status
+type ExitError struct {
+	msg        string
+	exitStatus int
+}
+
+func (e ExitError) Error() string {
+	return e.msg
+}
+
+func (e ExitError) ExitStatus() int {
+	return e.exitStatus
+}