glfw: re-instate prong

Bring back the 'c.GLFW_NOT_INITIALIZED' prong in 'convertError', such that if it is ever passed that error code, we can differentiate it from just an invalid input; because it is a valid input, we just guarantee that it won't occur.

glfw/src/errors.zig

@@ -84,6 +84,7 @@ pub const Error = error{
 fn convertError(e: c_int) Error!void {
     return switch (e) {
         c.GLFW_NO_ERROR => {},
+        c.GLFW_NOT_INITIALIZED => unreachable,
         c.GLFW_NO_CURRENT_CONTEXT => Error.NoCurrentContext,
         c.GLFW_INVALID_ENUM => Error.InvalidEnum,
         c.GLFW_INVALID_VALUE => Error.InvalidValue,