title: KeyError: Failed to format this callback filepath
body: <p>I am trying to obtain the validation accuracy and the choose to save the model having the best accuracy.But after the 1st epoch,it is showing the checkpoint error.</p>
  
  <pre><code>filepath="tumor_detection-{epoch:02d}-{val_acc:.2f}.hdf5"
  
  # save the model with the best validation (development) accuracy till now
  checkpoint = ModelCheckpoint(filepath, monitor='val_acc', verbose=1, save_best_only=True, mode='max')
  
  %%time
  model.fit(xtrain,ytrain,batch_size=32,epochs=30,validation_data=(xval,yval),callbacks=[checkpoint])```
  
  It is then showing an error as follows:
  Epoch 1/30
  51/51 [==============================] - ETA: 0s - loss: 0.4651 - accuracy: 0.7725
  ---------------------------------------------------------------------------
  KeyError                                  Traceback (most recent call last)
  /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/callbacks.py in _get_file_path(self, epoch, logs)
     1243         # placeholders can cause formatting to fail.
  -&gt; 1244         return self.filepath.format(epoch=epoch + 1, **logs)
     1245       except KeyError as e:
  
  KeyError: 'val_acc'
  
  During handling of the above exception, another exception occurred:
  
  KeyError                                  Traceback (most recent call last)
  9 frames
  &lt;decorator-gen-60&gt; in time(self, line, cell, local_ns)
  
  &lt;timed eval&gt; in &lt;module&gt;()
  
  /usr/local/lib/python3.6/dist-packages/tensorflow/python/keras/callbacks.py in _get_file_path(self, epoch, logs)
     1245       except KeyError as e:
     1246         raise KeyError('Failed to format this callback filepath: "{}". '
  -&gt; 1247                        'Reason: {}'.format(self.filepath, e))
     1248     else:
     1249       # If this is multi-worker training, and this worker should not
  
  KeyError: 'Failed to format this callback filepath: "tumor_detection-{epoch:02d}-{val_acc:.2f}.hdf5". Reason: \'val_acc\'
  </code></pre>