cmd: exit UX tweak

[gsora]

Implement several UX tweaks on the charon exit subcommands:

• Instead of printing "Fatal error: could not load keystore: no keys found", add some more context and point the user to the culprit of the issue: "Fatal error: could not load keystore, check if path exists: no keys found {"path": ".charon/validator_keys"}".
• Add [REQUIRED] suffix to all the required CLI flags.
• Clarified what's the purpose of fetch and broadcast.

category: misc
ticket: none

[codecov]

Codecov Report

Attention: Patch coverage is 5.88235% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 54.62%. Comparing base (fff580a) to head (53d928f).
Report is 1 commits behind head on main.

Files	Patch %	Lines
cmd/exit.go	0.00%	14 Missing ⚠️
cmd/exit_broadcast.go	0.00%	1 Missing ⚠️
cmd/exit_fetch.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3057      +/-   ##
==========================================
- Coverage   54.63%   54.62%   -0.01%     
==========================================
  Files         207      207              
  Lines       29145    29150       +5     
==========================================
+ Hits        15922    15924       +2     
- Misses      11383    11387       +4     
+ Partials     1840     1839       -1     

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud