Add glfwGetError

Related to #970.

If you have opinions on the design or implementation of this function,
please come join us in #970 before it is frozen for release.

docs/news.dox

@@ -4,6 +4,15 @@
 
 @section news_33 New features in 3.3
 
+
+@subsection news_33_geterror Error query
+
+GLFW now supports querying the last error code for the calling thread with @ref
+glfwGetError.
+
+@see @ref error_handling
+
+
 @subsection news_33_maximize Window maximization callback
 
 GLFW now supports window maximization notifications with @ref