Add traceback of exception during measurement to dataset's metadata

[astafan8]

The metadata key is called "exception".

questions/todos:

• is the name of the metadata key good? - yes, "measurement_exception"
• is storing a string good enough or shall we store the exception data in a more structured way? and if so, how? - string formatted by python's traceback module should be good enough for a start
• add a test :) - done

[jenshnielsen]

Cool, this has some suggestions of how to serialize that may be useful https://stackoverflow.com/questions/45240549/how-to-serialize-an-exception/45241491

[jenshnielsen]

I don't have a better idea for the key name

[codecov]

Codecov Report

Merging #2327 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2327      +/-   ##
==========================================
+ Coverage   62.12%   62.13%   +0.01%     
==========================================
  Files         200      200              
  Lines       26162    26164       +2     
==========================================
+ Hits        16252    16257       +5     
+ Misses       9910     9907       -3