readall: fail on Ruby syntax warnings

Previously, syntax warnings were printed, but didn't cause `readall` to
exit with a non-zero exit code. Now they do, making it easier to catch
accidentally introduced syntax warnings in the test bot.

Library/Homebrew/cmd/readall.rb

@@ -22,7 +22,8 @@ module Homebrew
           Thread.new do
             begin
               while rb = ruby_files.pop(true)
-                failed = true unless system RUBY_PATH, "-c", "-w", rb
+                # As a side effect, print syntax errors/warnings to `$stderr`.
+                failed = true if syntax_errors_or_warnings?(rb)
               end
             rescue ThreadError # ignore empty queue error
             end
@@ -69,4 +70,17 @@ module Homebrew
       end
     end
   end
+
+  private
+
+  def syntax_errors_or_warnings?(rb)
+    # Retrieve messages about syntax errors/warnings printed to `$stderr`, but
+    # discard a `Syntax OK` printed to `$stdout` (in absence of syntax errors).
+    messages = Utils.popen_read("#{RUBY_PATH} -c -w #{rb} 2>&1 >/dev/null")
+    $stderr.print messages
+
+    # Only syntax errors result in a non-zero status code. To detect syntax
+    # warnings we also need to inspect the output to `$stderr`.
+    !$?.success? || !messages.chomp.empty?
+  end
 end