Remove 'Stack trace' from break loop messages

[fingolfin]

I've added this in PR #6257 but now am not so sure anymore I like it, and the way it is inserted. See this comment on PR #6416.

[ThomasBreuer]

I thought the format was inspired by that of the error messages in Julia, where the specific message text is followed by a line Stacktrace: (not Stack trace:) and then the description of the call stack, with numbers for the levels.

I have no preference for keeping the line in question or leaving it out.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.87%. Comparing base (9196a1f) to head (a51536f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6429   +/-   ##
=======================================
  Coverage   78.87%   78.87%           
=======================================
  Files         685      685           
  Lines      293543   293541    -2     
  Branches     8679     8682    +3     
=======================================
+ Hits       231519   231520    +1     
+ Misses      60215    60212    -3     
  Partials     1809     1809           

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