Broken functionality because of missing dependency should be shown in GUI

[gdementen]

And not only in an exception. For example, if one tried to export to Excel without xlwings installed (see larray-project/larray#1128), we raise an helpful exception but a user using the GUI would never see it.

[gdementen]

In fact, the user-visible error message was already foreseen and this is apparently fixed in the devel branch already without changing the "outer function" with the error message code 😉 . The problem is that the exception raised (Exception) was not the same as the Exception caught (ImportError):

Traceback (most recent call last):
  File "C:\soft\Miniconda3x64\envs\dev313\Lib\site-packages\larray_editor\arraywidget.py", line 1097, in to_excel
    self.data_adapter.to_excel(raw_data, axes_names, vlabels, hlabels)
    ~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\soft\Miniconda3x64\envs\dev313\Lib\site-packages\larray_editor\arrayadapter.py", line 373, in to_excel
    wb = la.open_excel()
  File "C:\soft\Miniconda3x64\envs\dev313\Lib\site-packages\larray\inout\xw_excel.py", line 643, in open_excel
    raise Exception("open_excel() is not available because xlwings is not installed")
Exception: open_excel() is not available because xlwings is not installed

[gdementen]

merged in #299