Handle some uncaught exceptions in explain mode

[yonatan]

Explain mode passes some user input directly to stoi() and friends, which makes it crash on user errors/typos.

For example:

path
===============
1	2
1	3
1	4
1	5
2	3
2	4
2	5
3	4
3	5
4	5
===============

Explain is invoked.
Enter command > explain path("asd", 123)
stoll
*CRASH*

Explain is invoked.
Enter command > explainnegation path(1,6)
1: path(x,y) :- 
   edge(x,y).

2: path(x,z) :- 
   edge(x,y),
   path(y,z).

Pick a rule number: xcvb
stoi
*CRASH*

And with this patch:

Enter command > explain path(1,"Asd")
Invalid argument "Asd" for type 'i'
Enter command > explainnegation path(1,6)
1: path(x,y) :- 
   edge(x,y).

2: path(x,z) :- 
   edge(x,y),
   path(y,z).

Pick a rule number: asd
Invalid rule number
Enter command > 

[codecov]

Codecov Report

Attention: Patch coverage is 47.72727% with 23 lines in your changes missing coverage. Please review.

Project coverage is 81.06%. Comparing base (94ccab0) to head (8cd7e04).
Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
src/include/souffle/provenance/Explain.h	50.00%	15 Missing ⚠️
src/include/souffle/provenance/ExplainProvenance.h	42.85%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2539      +/-   ##
==========================================
- Coverage   81.14%   81.06%   -0.09%     
==========================================
  Files         492      492              
  Lines       33372    33392      +20     
==========================================
- Hits        27080    27068      -12     
- Misses       6292     6324      +32     

Files with missing lines	Coverage Δ
src/include/souffle/provenance/ExplainProvenance.h	83.33% <42.85%> (-5.96%)	⬇️
src/include/souffle/provenance/Explain.h	46.25% <50.00%> (-1.14%)	⬇️

... and 5 files with indirect coverage changes

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[quentin]

Hi, @yonatan it would be nice to have a few tests covering this improvement.

[yonatan]

True, but I probably won't get around to it