Fix a -Wcatch-value warning reported by GCC 8.1

```
src/translation.cpp:43:16: warning: interception du type polymorphique « class std::out_of_range » par valeur [-Wcatch-value=]
  } catch (std::out_of_range) {
```·

src/translation.cpp

@@ -40,7 +40,7 @@ const std::wstring &Translations::getTranslation(
 	std::wstring key = textdomain + L"|" + s;
 	try {
 		return m_translations.at(key);
-	} catch (std::out_of_range) {
+	} catch (const std::out_of_range &) {
 		warningstream << "Translations: can't find translation for string \""
 		              << wide_to_utf8(s) << "\" in textdomain \""
 		              << wide_to_utf8(textdomain) << "\"" << std::endl;
@@ -147,4 +147,4 @@ void Translations::loadTranslation(const std::string &data)
 
 		m_translations[textdomain + L"|" + oword1] = oword2;
 	}
-}
+}