fix(lib): add a more meaningful error message (#30)

* fix(lib): add a more meaningful error message

This adds an error message to explain why calling `self.pause()` cannot work if no previous animation was played.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: jeertmans

manim_slides/config.py

@@ -84,6 +84,11 @@ def number_is_strictly_posint(cls, v: int):
     def start_animation_is_before_end(cls, values):
         if values["start_animation"] >= values["end_animation"]:
 
+            if values["start_animation"] == values["end_animation"] == 0:
+                raise ValueError(
+                    "You have to play at least one animation (e.g., `self.wait()`) before pausing. If you want to start paused, use the approriate command-line option when presenting."
+                )
+
             raise ValueError(
                 "Start animation index must be strictly lower than end animation index"
             )