Made gl_set_swap_interval() return Result, not bool

Resolves #755

Author: icefoxen

src/sdl2/video.rs

@@ -791,8 +791,13 @@ impl VideoSubsystem {
         }
     }
 
-    pub fn gl_set_swap_interval<S: Into<SwapInterval>>(&self, interval: S) -> bool {
-        unsafe { sys::SDL_GL_SetSwapInterval(interval.into() as c_int) == 0 }
+    pub fn gl_set_swap_interval<S: Into<SwapInterval>>(&self, interval: S) -> Result<(), String> {
+        let result = unsafe { sys::SDL_GL_SetSwapInterval(interval.into() as c_int) };
+        if result == 0{
+            Ok(())
+        } else {
+            Err(get_error())
+        }
     }
 
     pub fn gl_get_swap_interval(&self) -> SwapInterval {