More verbose messages when failing to load render engine

[chapulina]

I'm working on this as part of gazebosim/gz-sensors#18, but the current state of this PR applies even outside that context.

Summary

This is making error messages more verbose when loading a render engine:

• If no render engine interface was found, but other interfaces were, print the names of the other interfaces.
• If multiple render engines were found, warn the user that there were others, and keep choosing a random one (note that pluginNames is an unordered_set, so getting begin is "random").

Test it

Checklist

• Signed all commits for DCO
• Added tests
• Added example world and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[codecov]

Codecov Report

Merging #236 (9c5ba35) into ign-rendering3 (36bc282) will decrease coverage by 0.03%.
The diff coverage is 33.33%.

@@                Coverage Diff                 @@
##           ign-rendering3     #236      +/-   ##
==================================================
- Coverage           50.63%   50.59%   -0.04%     
==================================================
  Files                 129      129              
  Lines               11864    11877      +13     
==================================================
+ Hits                 6007     6009       +2     
- Misses               5857     5868      +11     

Impacted Files	Coverage Δ
src/RenderEngineManager.cc	74.20% <33.33%> (-3.68%)	⬇️

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[ahcorde]

can we retarget this to a lower ign-rendering version instead of main ?

[chapulina]

can we retarget this to a lower ign-rendering version instead of main ?

Yeah I can do that 👍 I'll just get ign-sensors working with ign-plugin first to see if I need more changes here before retargeting.

[chapulina]

Retargeted to ign-rendering3 and opened for review.