Correctly detect when gem is not available

[christophebedard]

Relates to #821

Currently, when trying to install dependencies from the gem installer when gem isn't installed, we get this:

$ rosdep install --rosdistro rolling --from-paths src/test_pkg_gem

ERROR: Rosdep experienced an internal error.
Please go to the rosdep page [1] and file a bug report with the message below.
[1] : http://www.ros.org/wiki/rosdep

rosdep version: 0.21.0

Bad installer [gem]: [Errno 2] No such file or directory: 'gem'

This is with a workspace that has a single package.xml file with a single gem dependency: <depend>metaruby</depend>. The error message isn't really clear; users shouldn't really be asked to open an issue in this case.

This fix is just doing what the pip installer does: if gem isn't installed, then none of the resolved packages are currently installed on the system, so we can simply return right away. Later on, when we try to install them, we finally get to raise an error saying that gem isn't installed (in GemInstaller.get_install_command()). We get this:

$ rosdep install --rosdistro rolling --from-paths src/test_pkg_gem
ERROR: the following rosdeps failed to install
  gem: gem is not installed

This is the same as #822.

Signed-off-by: Christophe Bedard [email protected]

[codecov-commenter]

Codecov Report

Merging #823 (371e984) into master (211c5c6) will increase coverage by 0.07%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #823      +/-   ##
==========================================
+ Coverage   74.84%   74.92%   +0.07%     
==========================================
  Files          42       42              
  Lines        3304     3306       +2     
==========================================
+ Hits         2473     2477       +4     
+ Misses        831      829       -2     

Impacted Files	Coverage Δ
src/rosdep2/platforms/gem.py	84.61% <50.00%> (+6.23%)	⬆️

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[nuclearsandwich]

Thanks for these changes @christophebedard!