The --quiet should still display warnings

The is the previous intentional behaviour until
rubygems/bundler@ca0676c.

In my opinion, that previous behaviour was better and should be
restored, because we want our users to always see warnings and fix them.
And the original issue that motivated the change is fixable by other
means, namely through `BUNDLE_SILENCE_ROOT_WARNING`, or through
`BUNDLE_SILENCE_DEPRECATIONS` in general. Finally, the --quiet option is
still documented as "only print errors and warnings".

So this PR essentially reverts
rubygems/bundler@ca0676c
for the above reasons.

bundler/lib/bundler/cli/cache.rb

@@ -9,7 +9,7 @@ def initialize(options)
     end
 
     def run
-      Bundler.ui.level = "error" if options[:quiet]
+      Bundler.ui.level = "warn" if options[:quiet]
       Bundler.settings.set_command_option_if_given :path, options[:path]
       Bundler.settings.set_command_option_if_given :cache_path, options["cache-path"]
 

bundler/lib/bundler/cli/doctor.rb

@@ -61,7 +61,7 @@ def check!
     end
 
     def run
-      Bundler.ui.level = "error" if options[:quiet]
+      Bundler.ui.level = "warn" if options[:quiet]
       Bundler.settings.validate!
       check!
 

bundler/lib/bundler/cli/install.rb

@@ -8,7 +8,7 @@ def initialize(options)
     end
 
     def run
-      Bundler.ui.level = "error" if options[:quiet]
+      Bundler.ui.level = "warn" if options[:quiet]
 
       warn_if_root
 

bundler/lib/bundler/cli/update.rb

@@ -9,7 +9,7 @@ def initialize(options, gems)
     end
 
     def run
-      Bundler.ui.level = "error" if options[:quiet]
+      Bundler.ui.level = "warn" if options[:quiet]
 
       Plugin.gemfile_install(Bundler.default_gemfile) if Bundler.feature_flag.plugins?
 

bundler/spec/commands/install_spec.rb

@@ -585,7 +585,20 @@
   end
 
   describe "when requesting a quiet install via --quiet" do
-    it "should be quiet" do
+    it "should be quiet if there are no warnings" do
+      bundle "config set force_ruby_platform true"
+
+      gemfile <<-G
+        source "#{file_uri_for(gem_repo1)}"
+        gem 'rack'
+      G
+
+      bundle :install, :quiet => true
+      expect(out).to be_empty
+      expect(err).to be_empty
+    end
+
+    it "should still display warnings and errors" do
       bundle "config set force_ruby_platform true"
 
       create_file("install_with_warning.rb", <<~RUBY)
@@ -611,8 +624,9 @@ def run
       G
 
       bundle :install, :quiet => true, :raise_on_error => false, :env => { "RUBYOPT" => "-r#{bundled_app("install_with_warning.rb")}" }
+      expect(out).to be_empty
       expect(err).to include("Could not find gem 'non-existing-gem'")
-      expect(err).not_to include("BOOOOO")
+      expect(err).to include("BOOOOO")
     end
   end