bgcode: Propagate errors from valid_for_print

If a file fails the valid for print check not because it is still not
downloaded, but because it is somehow broken, report it as such. This
goes all the way to the GUI, instead of the „Downloading“ screen.

BFW-5830.

src/common/gcode/gcode_reader_binary.cpp

@@ -629,6 +629,25 @@ bool PrusaPackGcodeReader::valid_for_print() {
         return IterateResult_t::Continue;
     });
 
+    if (auto err = std::get_if<Result_t>(&res); err != nullptr) {
+        switch (*err) {
+        case Result_t::RESULT_EOF:
+            set_error(N_("File doesn't contain any print instructions"));
+            break;
+        case Result_t::RESULT_CORRUPT:
+            set_error(N_("File corrupt"));
+            break;
+        case Result_t::RESULT_ERROR:
+            set_error(N_("Unknown file error"));
+            break;
+        default:
+            // All the rest (OK, Timeout, out of range) don't prevent this
+            // file from being printable in the future, so don't set any
+            // error.
+            break;
+        }
+    }
+
     return std::holds_alternative<BlockHeader>(res);
 }