Fix failure to load a blend file behaving as if a new file is loaded

When a blend file failed to load from the command line,
blender would load a blend file with the path set to the location
that could not be loaded.

Now the command line argument is only treated as a new file
if the file does not already exist. Failure to load a file that
is found always exists with an error.

source/creator/creator_args.c

@@ -2089,20 +2089,38 @@ static int arg_handle_load_file(int UNUSED(argc), const char **argv, void *data)
       return -1;
     }
 
-    if (BKE_blendfile_extension_check(filepath)) {
-      /* Just pretend a file was loaded, so the user can press Save and it'll
-       * save at the filepath from the CLI. */
-      STRNCPY(G_MAIN->filepath, filepath);
-      printf("... opened default scene instead; saving will write to: %s\n", filepath);
+    const char *error_msg = NULL;
+    if (BLI_exists(filepath)) {
+      /* When a file is found but can't be loaded, handling it as a new file
+       * could cause it to be unintentionally overwritten (data loss).
+       * Further this is almost certainly not that a user would expect or want.
+       * If they do, they can delete the file beforehand. */
+      error_msg = "file could not be loaded";
     }
-    else {
-      fprintf(
-          stderr,
-          "Error: argument has no '.blend' file extension, not using as new file, exiting! %s\n",
-          filepath);
+    else if (!BKE_blendfile_extension_check(filepath)) {
+      /* Unrelated arguments should not be treated as new blend files. */
+      error_msg = "argument has no '.blend' file extension, not using as new file";
+    }
+
+    if (error_msg) {
+      fprintf(stderr, "Error: %s, exiting! %s\n", error_msg, filepath);
+
       G.is_break = true;
       WM_exit(C);
+      /* Unreachable, return for clarity. */
+      return -1;
     }
+
+    /* Behave as if a file was loaded, calling "Save" will write to the `filepath` from the CLI.
+     *
+     * WARNING: The path referenced may be incorrect, no attempt is made to validate the path
+     * here or check that writing to it will work. If the users enters the path of a directory
+     * that doesn't exist (for e.g.) saving will fail.
+     * Attempting to create the file at this point is possible but likely to cause more
+     * trouble than it's worth (what with network drives), removable devices ... etc. */
+
+    STRNCPY(G_MAIN->filepath, filepath);
+    printf("... opened default scene instead; saving will write to: %s\n", filepath);
   }
 
   return 0;