Adjust when 'Stack trace:' message is printed

[fingolfin]

Alternative to and hence closes #6429.

This retains the Stack trace: but now prints it more consistently (e.g. also when the browse package is used), and doesn't print it in pointless cases (when there is no stack trace)

[fingolfin]

This duplicates an example from debug.xml. Probably would be better to remove it here, and instead insert a link to the relevant section in debug.xml

[ThomasBreuer]

doc/tut/migrat.xml is not part of the Tutorial and should perhaps better be removed, cf. #6431.

[fingolfin]

Ah I did not realize, thanks.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.87%. Comparing base (8187e9e) to head (ae886cb).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/error.g	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6432      +/-   ##
==========================================
- Coverage   78.87%   78.87%   -0.01%     
==========================================
  Files         685      685              
  Lines      293550   293543       -7     
  Branches     8672     8672              
==========================================
- Hits       231530   231521       -9     
- Misses      60211    60213       +2     
  Partials     1809     1809              

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[lgoettgens]

I prefer this over #6429. Maybe also print "Stacktrace:" instead of "Stack trace:"?

[ThomasBreuer]

This looks good:
Do not print Stack trace if no stack follows,
but otherwise separate the error message from the numbered stack trace with a Stack trace line.

[fingolfin]

@lgoettgens I use "Stack trace" because that spelling (with a space) is how the GAP manual has spelled it for many years, and arguably it is the correct spelling (no compound words in English, unlike German).