Revise some error messages

[fingolfin]

• Don't use trailing period
• Don't print values; the user can print them if they want to via the break loop
• Be more consistent in how errors are worded (more to be done)

[codecov[bot]]

Codecov Report

Merging #83 (6059877) into main (d698214) will increase coverage by 1.19%. The diff coverage is 29.41%.

@@            Coverage Diff             @@
##             main      #83      +/-   ##
==========================================
+ Coverage   92.36%   93.56%   +1.19%     
==========================================
  Files          14       14              
  Lines        2514     2485      -29     
==========================================
+ Hits         2322     2325       +3     
+ Misses        192      160      -32     

Impacted Files	Coverage Δ
gap/ClassicalMaximals.gi	84.66% <0.00%> (+0.43%)	:arrow_up:
gap/ClassicalNormalizerMatrixGroups.gi	93.54% <0.00%> (ø)
gap/ExtraspecialNormalizerMatrixGroups.gi	96.74% <0.00%> (+1.41%)	:arrow_up:
gap/Forms.gi	89.13% <0.00%> (+0.95%)	:arrow_up:
gap/SemilinearMatrixGroups.gi	95.23% <0.00%> (+2.52%)	:arrow_up:
gap/TensorInducedMatrixGroups.gi	98.11% <0.00%> (+1.81%)	:arrow_up:
gap/Utils.gi	94.03% <0.00%> (+0.85%)	:arrow_up:
gap/SubfieldMatrixGroups.gi	97.79% <60.00%> (+4.24%)	:arrow_up:
gap/ImprimitiveMatrixGroups.gi	98.20% <62.50%> (+1.16%)	:arrow_up:
gap/ReducibleMatrixGroups.gi	99.40% <66.66%> (+0.59%)	:arrow_up:
... and 1 more