2D: add bounds back to the stack if the runner failed

This bug manifested when the function failed after starting the
runner. For example the function is calling a function that is not
defined on the engines will lead to a NameError, then after importing
that on the engines, you wouldn't be able to start the runner again,
because the learner would be in a bad state. This commit fixes that
issue.

adaptive/learner/learner2D.py

@@ -368,6 +368,9 @@ class Learner2D(BaseLearner):
 
     def remove_unfinished(self):
         self._interp = set()
+        for p in self._bounds_points:
+            if p not in self.data:
+                self._stack[p] = np.inf
 
     def plot(self, n=None, tri_alpha=0):
         hv = ensure_holoviews()