Implement informative error messages if the needed columns are missing

[valentinitnelav]

If any of these columns is missing then one gets a cryptic error message at the moment: windows_img_path, id_box, x, y, width & height.

Example of a current not-user-friendly error message when the column windows_img_path is missing (e.g. renamed by accident, or simply missing):

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Error
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Traceback (most recent call last):
  File "c:\python38\lib\site-packages\pandas\core\indexes\base.py", line 2895, in get_loc
    return self._engine.get_loc(casted_key)
  File "pandas\_libs\index.pyx", line 70, in pandas._libs.index.IndexEngine.get_loc
  File "pandas\_libs\index.pyx", line 101, in pandas._libs.index.IndexEngine.get_loc
  File "pandas\_libs\hashtable_class_helper.pxi", line 1675, in pandas._libs.hashtable.PyObjectHashTable.get_item
  File "pandas\_libs\hashtable_class_helper.pxi", line 1683, in pandas._libs.hashtable.PyObjectHashTable.get_item
KeyError: 'windows_img_path'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\...\field-img-taxa-annotation\vba_python_img_with_box\xlwings_test\xlwings_test_project.py", line 68, in main
    display_img()
  File "C:\...\field-img-taxa-annotation\vba_python_img_with_box\xlwings_test\xlwings_test_project.py", line 44, in display_img
    img_path = df['windows_img_path'][0]
  File "c:\python38\lib\site-packages\pandas\core\frame.py", line 2902, in __getitem__
    indexer = self.columns.get_loc(key)
  File "c:\python38\lib\site-packages\pandas\core\indexes\base.py", line 2897, in get_loc
    raise KeyError(key) from err
KeyError: 'windows_img_path'

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OK   
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