Fix CURLException::what().

src/curl_wrapper.cpp

@@ -222,15 +222,19 @@ void CURLWrapper::check(const CURLcode code)
     }
 }
 
-const char *CURLException::what() noexcept
+const char *CURLException::what() const noexcept
 {
-    if (!_error_message.empty())
+    // NOTE: The string has to be static, or it'll vanish before it can be
+    //       used. Couldn't find good documentation on that.
+    static string error_string;
+    error_string = _error_message;
+    if (!error_string.empty())
     {
-        _error_message = ": " + _error_message;
+        error_string = " – " + error_string;
     }
-    _error_message = "libcurl_error: " + std::to_string(error_code)
-                     + _error_message;
-    return _error_message.c_str();
+    error_string = "libcurl error: " + std::to_string(error_code)
+                   + error_string;
+    return error_string.c_str();
 }
 
 } // namespace curl_wrapper

src/curl_wrapper.hpp

@@ -233,7 +233,7 @@ public:
      *
      *  @since  0.1.0
      */
-    [[nodiscard]] const char *what() noexcept;
+    [[nodiscard]] const char *what() const noexcept override;
 
 private:
     string _error_message;