[rubygems/rubygems] Improve error messages in gem helpers

Previously they were printing the original command that was run, and
telling the user to rerun it. However, the command sometimes would not
match the exact command that was run (for example, when using the
`--local` flag), and in any case, it's simpler and more useful to print
the underlying error anyways.

https://github.com/rubygems/rubygems/commit/5bc0d51b58

lib/bundler/gem_helper.rb

@@ -98,9 +98,9 @@ module Bundler
       built_gem_path ||= build_gem
       cmd = [*gem_command, "install", built_gem_path.to_s]
       cmd << "--local" if local
-      _, status = sh_with_status(cmd)
+      out, status = sh_with_status(cmd)
       unless status.success?
-        raise "Couldn't install gem, run `gem install #{built_gem_path}' for more detailed output"
+        raise("Running `#{cmd}` failed with the following output:\n\n#{out}\n")
       end
       Bundler.ui.confirm "#{name} (#{version}) installed."
     end
@@ -219,7 +219,7 @@ module Bundler
       out, status = sh_with_status(cmd, &block)
       unless status.success?
         cmd = cmd.shelljoin if cmd.respond_to?(:shelljoin)
-        raise(out.empty? ? "Running `#{cmd}` failed. Run this command directly for more detailed output." : out)
+        raise("Running `#{cmd}` failed with the following output:\n\n#{out}\n")
       end
       out
     end

spec/bundler/bundler/gem_helper_spec.rb

@@ -219,7 +219,7 @@ RSpec.describe Bundler::GemHelper do
             FileUtils.touch app_gem_path
             app_gem_path
           end
-          expect { subject.install_gem }.to raise_error(/Couldn't install gem/)
+          expect { subject.install_gem }.to raise_error(/Running `#{gem_bin} install #{app_gem_path}` failed/)
         end
       end
     end