Handle some uncaught exceptions in explain mode - #2539

path
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1	2
1	3
1	4
1	5
2	3
2	4
2	5
3	4
3	5
4	5
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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*

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 >