Make getters raise KeyError instead of returning "NotExist"

[ondras12345]

This closes #260

I chose to use KeyError over ModelicaSystemError, because it makes the implementation easier (dicts already raise it for us), and because I find it useful to be able to distinguish invalid input from other errors. If you prefer ModelicaSystemError, let me know and I'll change it.

[syntron]

@ondras12345 I tried to use project specific exceptions if possible (and meaningful); thus, on an exception I could catch these to ensure there is no project related error.

My changes to the code are based on this assumption; i.e. raise ModelicaSystemError in ModelicaSystem.py and OMCSessionException in OMCSession.py if the case is due to OMC or the related Python code.

[ondras12345]

on an exception I could catch these to ensure there is no project related error

@syntron I'm not sure I understand what you mean by this. Could you please write a short demo?

In my mind, if a user writes mod.getParameters("badParameter"), it isn't much different from doing {}["badKey"] in his own code.
KeyError has a clear meaning: you tried to get a value that does not exist in the collection.
ModelicaSystemError could mean many different things (model failed to simulate due to running out of RAM, etc.).
Raising ModelicaSystemError feels like raising bare Exception instead of KeyError.

A similar situation in numpy:

In [1]: import numpy as np
In [2]: A = np.array([[1,2],[3,4]])
In [3]: A[1,2]
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
Cell In[3], line 1
----> 1 A[1,2]

IndexError: index 2 is out of bounds for axis 1 with size 2

It raises a standard IndexError rather than a custom exception.

If you want to make sure that the exception you are catching came from OMPython, you should probably only put the commands that use OMPython in the try ... except block, and keep your code out of it.

However, this is all just my opinion, and I'm open to changing it if you show me a convincing use case / logical reason.

[syntron]

@ondras12345 I'm fine with your approach!

My way is focused on the possibility to catch all errors related to ModelicaSystem by this one exception; this would mean that a check for this would ensure that the functionality of ModelicaSystem is covert.