Report possible error prior checking output of gem uninstall

[voxik]

Originally, the failed test case reported following error:

Failure: test_gem_exec_gem_uninstall(TestGemCommandsExecCommand):
  <""> was expected to include
  <"Successfully uninstalled a-2\n">.
/mnt/test/rubygems/test_gem_commands_exec_command.rb:742:in `block in test_gem_exec_gem_uninstall'
     739:
     740:       # assert_empty @ui.error
     741:       refute_includes @ui.output, "running gem exec with"
  => 742:       assert_includes @ui.output, "Successfully uninstalled a-2\n"
     743:
     744:       invoke "--verbose", "gem", "uninstall", "b"
     745:       assert_includes @ui.output, "Successfully uninstalled b-2\n"
/mnt/lib/rubygems/user_interaction.rb:46:in `use_ui'
/mnt/lib/rubygems/user_interaction.rb:69:in `use_ui'
/mnt/test/rubygems/test_gem_commands_exec_command.rb:726:in `test_gem_exec_gem_uninstall'

This does not tell much. Empty string is more often good sign, but not in this case. However, checking error output first helps with understanding possible issue:

Failure: test_gem_exec_gem_uninstall(TestGemCommandsExecCommand):
  <"ERROR:  While executing gem ... (Gem::FilePermissionError)\n" +
  "    You don't have write permissions for the /builddir/bin directory.\n" +
  "\t/mnt/lib/rubygems/uninstaller.rb:213:in `remove_executables'\n" +

... snip ...

/mnt/test/rubygems/test_gem_commands_exec_command.rb:740:in `block in test_gem_exec_gem_uninstall'
     737:         nil
     738:       end
     739:
  => 740:       assert_empty @ui.error
     741:       refute_includes @ui.output, "running gem exec with"
     742:       assert_includes @ui.output, "Successfully uninstalled a-2\n"
     743:
/mnt/lib/rubygems/user_interaction.rb:46:in `use_ui'
/mnt/lib/rubygems/user_interaction.rb:69:in `use_ui'
/mnt/test/rubygems/test_gem_commands_exec_command.rb:726:in `test_gem_exec_gem_uninstall'

BTW I have hit this in the context of #7119. This issue is caused by operating_system.rb overriding Gem.operating_system_defaults method and explicitly adding --bindir option.

[deivid-rodriguez]

👍