Fix error handling deadlock.
Flush errors upon entering the error callback.
Warn users to report printed deadlock errors.
Fix typo "an uncaught.." -> "An uncaught..".
Merge pull request #94 from go-gl/alt_fix_errors_deadlock

Fix error handling deadlock.
Merge branch 'devel_glfw3.1' of https://github.com/go-gl/glfw3