Backport cmdsim.rb: fix ruby syntax to ign-gazebo3

[scpeters]

Backport #1884. Part of #1867.

This replaces File.exists? calls with File.exist? to fix the UNIT_gz_TEST on macOS. It also removes a
rescue statement that was hiding useful error messages.

Note to maintainers: Remember to use Rebase-and-merge.

[iche033]

hmm CmdLine.CachedFuelWorld is failing on ubuntu builds, not sure if it's related. Retriggered a couple of github actions

[scpeters]

hmm CmdLine.CachedFuelWorld is failing on ubuntu builds, not sure if it's related. Retriggered a couple of github actions

maybe it will be fixed once gazebosim/gz-fuel-tools#318 is released?

[scpeters]

hmm CmdLine.CachedFuelWorld is failing on ubuntu builds, not sure if it's related. Retriggered a couple of github actions

maybe it will be fixed once gazebosim/gz-fuel-tools#318 is released?

I tested locally, and that appears to fix it. I've opened a PR to make a patch release of ign-fuel-tools 4.8.1 gazebosim/gz-fuel-tools#325

[scpeters]

I just made the ign-fuel-tools 4.8.1 release and will restart CI

[scpeters]

the fuel tools 4.8.1 release didn't seem to fix the CmdLine.CachedFuelWorld test

[scpeters]

the fuel tools 4.8.1 release didn't seem to fix the CmdLine.CachedFuelWorld test

I think we also need to backport 6acac79 from #1771

[scpeters]

I believe the remaining CLI test is fixed by 34aadc8

[codecov]

Codecov Report

Merging #1885 (34aadc8) into ign-gazebo3 (1a61269) will increase coverage by 0.01%.
The diff coverage is 50.00%.

@@               Coverage Diff               @@
##           ign-gazebo3    #1885      +/-   ##
===============================================
+ Coverage        77.98%   78.00%   +0.01%     
===============================================
  Files              255      255              
  Lines            15041    15041              
===============================================
+ Hits             11730    11732       +2     
+ Misses            3311     3309       -2     

Impacted Files	Coverage Δ
include/ignition/gazebo/ServerConfig.hh	100.00% <ø> (ø)
src/ServerConfig.cc	90.28% <ø> (ø)
src/ServerPrivate.cc	82.60% <0.00%> (ø)
src/World.cc	96.87% <ø> (ø)
src/network/NetworkManagerPrimary.cc	78.89% <ø> (ø)
...rc/systems/ackermann_steering/AckermannSteering.hh	100.00% <ø> (ø)
src/systems/altimeter/Altimeter.cc	73.17% <ø> (-0.33%)	⬇️
src/gui/AboutDialogHandler.cc	70.00% <100.00%> (+3.33%)	⬆️
src/Server.cc	86.14% <0.00%> (+1.20%)	⬆️

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