Improve error handling in k-util.sh

[Baltoli]

This PR reorganises the execute function in k-util.sh:

• The amount of noise produced in verbose mode is reduced by only executing the actual command inside -x; all conditional tests are performed outside.
• The return code of the command is stored, and the existing error reporting function is used to report any failures to the user (even outside of verbose mode).

Tested locally to make sure the output is good. For example, by patching kore-expand-macros to just return 35:

+ kparse --definition ./test-kompiled --module TEST .krun-2022-09-30-12-37-54-1ys0guZ5YC/tmp.PGM.pretty.10LcVUXY1O
+ ret=0
+ llvm-krun -c PGM tmp.in.PGM.5P6NhEbdgW KItem korefile --directory /home/bruce/code/scratch/test-kompiled -v -save-temps --dry-run -nm -o tmp.in.npwbCwNMfZ
+ ret=0
+ kore-expand-macros ./test-kompiled .krun-2022-09-30-12-37-54-1ys0guZ5YC/tmp.in.npwbCwNMfZ
+ ret=35
[Error] krun: kore-expand-macros ./test-kompiled 
.krun-2022-09-30-12-37-54-1ys0guZ5YC/tmp.in.npwbCwNMfZ

Fixes #2817