Raise ValueError for unknown formats in the import/export OD APIs

[erlend-aasland]

Fixes #475

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All modified and coverable lines are covered by tests ✅

Project coverage is 67.68%. Comparing base (5734f37) to head (bb1f9e9).

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #476      +/-   ##
==========================================
+ Coverage   67.55%   67.68%   +0.12%     
==========================================
  Files          26       26              
  Lines        3101     3104       +3     
  Branches      521      521              
==========================================
+ Hits         2095     2101       +6     
+ Misses        864      862       -2     
+ Partials      142      141       -1     

Files	Coverage Δ
canopen/objectdictionary/__init__.py	82.29% <100.00%> (+1.16%)	⬆️

[acolomb]

This looks fine overall. But I still favor ValueError in these cases, as it is simply an invalid string being passed for the parameter. Such things should be sanitized / limited before calling the function IMHO. If it comes from a config file, ValueError will be a correct description of th failure cause. In contrast, ObjectDictionaryError concerns invalid operations using the OD, as its docstring states: """Unsupported operation with the current Object Dictionary."""

[erlend-aasland]

I'm fine with ValueError; it is a good alternative to ObjectDictionaryError.

[erlend-aasland]

Thanks a lot for the review and the helpful discussion; highly appreciated.