Fix system() exception showing regression

Rather than showing a backtrace that says "couldn't find command blah". Admittedly it's possible that the error will be something else, but unlikely. And this is neater.

Ideally we'd push the bt through an error pipe like we do with install.rb. And I guess we'll do this eventually.

Library/Homebrew/formula.rb

@@ -226,7 +226,8 @@ protected
         rd.close
         $stdout.reopen wr
         $stderr.reopen wr
-        exec cmd, *args
+        exec(cmd, *args) rescue nil
+        exit! 1 # never gets here unless exec threw or failed
       end
       wr.close
       out = ''

Library/Homebrew/utils.rb

@@ -88,8 +88,8 @@ end
 def safe_system cmd, *args
   puts "#{cmd} #{args*' '}" if ARGV.verbose?
   fork do
-    trap("EXIT") {} # no bt on exit from this short-lived fork
-    exit! 1 unless exec(cmd, *args)
+    exec(cmd, *args) rescue nil
+    exit! 1 # never gets here unless exec failed
   end
   Process.wait
   raise ExecutionError.new(cmd, args, $?) unless $?.success?