catch mt940-errors when getting transactions

[marcelb98]

Actually importing transactions is crashing if the MT940 sent by the bank is not correct. Depending on what was wrong in the MT940 you'll get another exception of the mt940 library. I've submitted some examples of malformed MT940 in wolph/mt940#65

This PR adds an alternative for the get_transactions() function which catches those exceptions.
It returns the statements which were correct and could be parsed on the one hand and the malformed MT940-strings coupled with the thrown exception on the other hand.

With those caught errors it's possible to review the MT940 and rerun the parser later.

[codecov]

Codecov Report

Merging #48 into master will decrease coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master      #48      +/-   ##
==========================================
- Coverage   89.17%   89.16%   -0.01%     
==========================================
  Files          23       23              
  Lines        2966     2982      +16     
==========================================
+ Hits         2645     2659      +14     
- Misses        321      323       +2

Impacted Files	Coverage Δ
fints/client.py	75% <87.5%> (+0.34%)	⬆️

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[raphaelm]

I think we can close this as the bug in mt940 seems to be fixed and this seems to be a very special-case API