[Changed] Raise an error if the yarn licenses list command fails

Previously, LicenseFinder::Yarn.new.current_packages silently returned
an empty array when `yarn licenses list` failed, resulting in no license
violations.
This is dangerous as it can lead to thinking there are no license issues
present.

Missing or incorrectly configured environment variables can lead to
`yarn licenses list` failing.

lib/license_finder/package_managers/yarn.rb

@@ -13,8 +13,8 @@ def current_packages
       suffix = " --cwd #{project_path}" unless project_path.nil?
       cmd += suffix unless suffix.nil?
 
-      stdout, _stderr, status = Cmd.run(cmd)
-      return [] unless status.success?
+      stdout, stderr, status = Cmd.run(cmd)
+      raise "Command '#{cmd}' failed to execute: #{stderr}" unless status.success?
 
       packages = []
       incompatible_packages = []

spec/lib/license_finder/package_managers/yarn_spec.rb

@@ -170,6 +170,14 @@ module LicenseFinder
           expect(subject.current_packages.first.homepage).to eq 'https://github.com/felixg?/node-stack-trace'
         end
       end
+
+      context "when the shell command raises an error" do
+        it "raises" do
+          allow(SharedHelpers::Cmd).to receive(:run).with(Yarn::SHELL_COMMAND + " --cwd #{Pathname(root)}").and_return([nil, 'error', cmd_failure])
+
+          expect { subject.current_packages }.to raise_error(%r{Command 'yarn licenses list --no-progress --json --cwd #{Pathname(root)}' failed to execute: error})
+        end
+      end
     end
 
     describe '.prepare_command' do