lock_file: better message if too many open files

Recommend using `ulimit -n` to increase maximum
number of open files.

Library/Homebrew/lock_file.rb

@@ -42,7 +42,11 @@ class LockFile
   def create_lockfile
     return if @lockfile.present? && !@lockfile.closed?
 
+    begin
       @lockfile = @path.open(File::RDWR | File::CREAT)
+    rescue Errno::EMFILE
+      odie "The maximum number of open files on this system has been reached. Use `ulimit -n` to increase this limit."
+    end
     @lockfile.fcntl(Fcntl::F_SETFD, Fcntl::FD_CLOEXEC)
   end
 end