Fix handling of missing CSM matrix

The code checks for the camera-stage mapper matrix by looking at its truth value.
However, this is ambiguous because the matrix has many elements.  That means that
if it's missing, we get the expected result, but if it's present
we get an error.

I have avoided dict.get() and instead look it up with [] notation.
This will raise a KeyError if it's missing, which I
handle helpfully.
If the key is present but the value is not castable to an array, we'll
get an exception anyway.

openflexure_microscope/api/default_extensions/camera_stage_mapping.py

@@ -194,10 +194,11 @@ class CSMExtension(BaseExtension):
     @property
     def image_to_stage_displacement_matrix(self) -> np.ndarray:  # 2x2 integer array
         """A 2x2 matrix that converts displacement in image coordinates to stage coordinates."""
-        displacement_matrix = self.get_settings().get("image_to_stage_displacement")
-        if not displacement_matrix:
+        settings = self.get_settings()
+        try:
+            return np.array(settings["image_to_stage_displacement"])
+        except KeyError:
             raise ValueError("The microscope has not yet been calibrated.")
-        return np.array(displacement_matrix)
 
     def move_in_image_coordinates(self, displacement_in_pixels: XYCoordinateType):
         """Move by a given number of pixels on the camera"""