Add helper to log Windows system errors

It will help to log errors returned by GetLastError() or
WSAGetLastError():
 - <https://docs.microsoft.com/en-us/windows/win32/api/errhandlingapi/nf-errhandlingapi-getlasterror>
 - <https://docs.microsoft.com/en-us/windows/win32/api/winsock/nf-winsock-wsagetlasterror>
 - <https://docs.microsoft.com/en-us/windows/win32/debug/system-error-codes#system-error-codes>

Always log the errors in English to be able to read them in bug reports.

app/src/util/log.c

@@ -1,5 +1,8 @@
 #include "log.h"
 
+#if _WIN32
+# include <windows.h>
+#endif
 #include <assert.h>
 
 static SDL_LogPriority
@@ -51,3 +54,24 @@ sc_get_log_level(void) {
     SDL_LogPriority sdl_log = SDL_LogGetPriority(SDL_LOG_CATEGORY_APPLICATION);
     return log_level_sdl_to_sc(sdl_log);
 }
+
+#ifdef _WIN32
+bool
+sc_log_windows_error(const char *prefix, int error) {
+    assert(prefix);
+
+    char *message;
+    DWORD flags = FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM;
+    DWORD lang_id = MAKELANGID(LANG_ENGLISH, SUBLANG_ENGLISH_US);
+    int ret =
+        FormatMessage(flags, NULL, error, lang_id, (char *) &message, 0, NULL);
+    if (ret <= 0) {
+        return false;
+    }
+
+    // Note: message already contains a trailing '\n'
+    LOGE("%s: [%d] %s", prefix, error, message);
+    LocalFree(message);
+    return true;
+}
+#endif

app/src/util/log.h

@@ -26,4 +26,10 @@ sc_set_log_level(enum sc_log_level level);
 enum sc_log_level
 sc_get_log_level(void);
 
+#ifdef _WIN32
+// Log system error (typically returned by GetLastError() or similar)
+bool
+sc_log_windows_error(const char *prefix, int error);
+#endif
+
 #endif

app/src/util/net.c

@@ -117,14 +117,7 @@ set_cloexec_flag(sc_raw_socket raw_sock) {
 static void
 net_perror(const char *s) {
 #ifdef _WIN32
-    int error = WSAGetLastError();
+    sc_log_windows_error(s, WSAGetLastError());
-    char *wsa_message;
-    FormatMessage(FORMAT_MESSAGE_ALLOCATE_BUFFER | FORMAT_MESSAGE_FROM_SYSTEM,
-            NULL, error, MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
-            (char *) &wsa_message, 0, NULL);
-    // no explicit '\n', wsa_message already contains a trailing '\n'
-    fprintf(stderr, "%s: [%d] %s", s, error, wsa_message);
-    LocalFree(wsa_message);
 #else
     perror(s);
 #endif