improvements for the group atlas

[ThomasBreuer]

• improve the error messages shown by atlas_group and atlas_subgroup in case of failed downloads of data at runtime (as discussed for Jaa/patch/is unimodular Nemocas/Nemo.jl#2054)
• add show_atlas_info (as discussed in Add more AtlasRep wrappers #1557)

Concerning show_atlas_info, now on gets the following, similar to the situation in GAP.

julia> show_atlas_info(["M11", "M12", "M22"])
group |  # | maxes | cl | cyc | out | fnd | chk | prs
------+----+-------+----+-----+-----+-----+-----+----
M11   | 45 |     5 |  + |  +  |     |  +  |  +  |  + 
M12   | 58 |    11 |  + |  +  |     |  +  |  +  |  + 
M22   | 58 |     8 |  + |  +  |     |  +  |  +  |  + 

julia> show_atlas_info("M11")
Representations for G = M11:    (all refer to std. generators 1)
----------------------------
 1: G <= Sym(11)                   4-trans., on cosets of A6.2_3 (1st max.)
 2: G <= Sym(12)                   3-trans., on cosets of L2(11) (2nd max.)
 3: G <= Sym(22)                   rank 3, on cosets of A6 < A6.2_3
 4: G <= Sym(55)                   rank 3, on cosets of 3^2:Q8.2 (3rd max.)
 5: G <= Sym(66)                   rank 4, on cosets of S5 (4th max.)
 6: G <= Sym(110)                  rank 4, on cosets of 3^2:8 < 3^2:Q8.2
 7: G <= Sym(144)                  rank 6, on cosets of 11:5 < L2(11)
 8: G <= Sym(165)                  rank 8, on cosets of 2.S4 (5th max.)
 9: G <= GL(10,2)                  character 10a
10: G <= GL(32,2)                  character 16ab
11: G <= GL(44,2)                  character 44a
12: G <= GL(5a,3)                  character 5a
13: G <= GL(5b,3)                  character 5b
14: G <= GL(10a,3)                 character 10a
15: G <= GL(10b,3)                 character 10b
16: G <= GL(10c,3)                 character 10c
17: G <= GL(24,3)                  character 24a
18: G <= GL(45,3)                  character 45a
19: G <= GL(16a,4)                 character 16a
20: G <= GL(16b,4)                 character 16b
21: G <= GL(10a,5)                 character 10a
22: G <= GL(11,5)                  character 11a
23: G <= GL(16a,5)                 character 16a
24: G <= GL(16b,5)                 character 16b
25: G <= GL(20,5)                  character 10bc
26: G <= GL(45,5)                  character 45a
27: G <= GL(55,5)                  character 55a
28: G <= GL(9,11)                  character 9a
29: G <= GL(10a,11)                character 10a
30: G <= GL(10b,11)                character 10b
31: G <= GL(11,11)                 character 11a
32: G <= GL(16,11)                 character 16a
33: G <= GL(44,11)                 character 44a
34: G <= GL(55,11)                 character 55a
35: G <= GL(10b,25)                character 10b
36: G <= GL(10c,25)                character 10c
37: G <= GL(10a,Z)                 character 10a
38: G <= GL(11,Z)                  character 11a
39: G <= GL(20,Z)                  character 10bc
40: G <= GL(32,Z)                  character 16ab
41: G <= GL(44,Z)                  character 44a
42: G <= GL(45,Z)                  character 45a
43: G <= GL(55,Z)                  character 55a
44: G <= GL(10b,Field([Sqrt(-2)])) character 10b
45: G <= GL(10c,Field([Sqrt(-2)])) character 10c

Programs for G = M11:    (all refer to std. generators 1)
---------------------
- presentation                                        
- repr. cyc. subg.                                    
- std. gen. finder                                    
- class repres.:
  (direct)                                            
  (composed)                                          
- maxes (all 5):
  1:  A6.2_3                                          
  1:  A6.2_3                                  (std. 1)
  2:  L2(11)                                          
  2:  L2(11)                                  (std. 1)
  3:  3^2:Q8.2                                        
  4:  S5                                              
  4:  S5                                      (std. 1)
  5:  2.S4                                            
- standardizations of maxes:
  from 1st max., version 1 to A6.2_3, std. 1          
  from 2nd max., version 1 to L2(11), std. 1          
  from 4th max., version 1 to A5.2, std. 1            
- std. gen. checker:
  (check)                                             
  (pres)             

(The output is a bit nicer if unicode is allowed.)