Added detail about why a file couldn't be opened (thanks mgerhardy!)

Fixes https://github.com/libsdl-org/SDL/issues/10484

(cherry picked from commit 15120133202da5980c17b4bf363b5fdb6f32de6b)

src/file/SDL_iostream.c

@@ -26,6 +26,7 @@
 
 #ifdef HAVE_STDIO_H
 #include <stdio.h>
+#include <errno.h>
 #include <sys/stat.h>
 #endif
 #ifdef HAVE_LIMITS_H
@@ -565,7 +566,7 @@ SDL_IOStream *SDL_IOFromFile(const char *file, const char *mode)
         FILE *fp = fdopen(fd, mode);
         if (!fp) {
             close(fd);
-            SDL_SetError("Unable to open file descriptor (%d) from URI %s", fd, file);
+            SDL_SetError("Unable to open file descriptor (%d) from URI %s: %s", fd, file, strerror(errno));
             return NULL;
         }
 
@@ -655,7 +656,7 @@ SDL_IOStream *SDL_IOFromFile(const char *file, const char *mode)
         #endif
 
         if (!fp) {
-            SDL_SetError("Couldn't open %s", file);
+            SDL_SetError("Couldn't open %s: %s", file, strerror(errno));
         } else if (!IsRegularFileOrPipe(fp)) {
             fclose(fp);
             fp = NULL;