SSH Agent: Show correct error messages in main window

[hifi]

Fixes #7152

The error checking code for main window agent operations were completely off and 100% useless, like the author of the code.

I said I'll disable the context menu but it looks a bit heavy to actually do all of the checks at that point. This fixes the error message, though.

Testing strategy

Adding and removing a key with attachment type selected but the attachment left empty.. Shouldn't require much more.

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 8 lines in your changes missing coverage. Please review.

Project coverage is 64.13%. Comparing base (296cbf0) to head (4510799).
Report is 557 commits behind head on develop.

Files with missing lines	Patch %	Lines
src/gui/DatabaseWidget.cpp	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7166      +/-   ##
===========================================
+ Coverage    64.12%   64.13%   +0.02%     
===========================================
  Files          335      335              
  Lines        42284    42288       +4     
===========================================
+ Hits         27111    27121      +10     
+ Misses       15173    15167       -6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[droidmonkey]

At least the author of the code can fix the problems when they are found. Estimating 45% usefulness 😄