Report lack of support for the subtract blendmode through the Blue Screen of LÖVE instead of stderr spam.

src/modules/graphics/opengl/Graphics.cpp

@@ -525,7 +525,7 @@ namespace opengl
 			}
 		} else {
 			if (mode == BLEND_SUBTRACTIVE) {
-				std::cerr << "This graphics card does not support the subtract blend mode!" << std::endl;
+				throw Exception("This graphics card does not support the subtract blend mode!");
 			}
 			// GL_FUNC_ADD is the default even without access to glBlendEquation, so that'll still work.
 		}

src/modules/graphics/opengl/wrap_Graphics.cpp

@@ -532,7 +532,14 @@ namespace opengl
 		if (!Graphics::getConstant(str, mode))
 			return luaL_error(L, "Invalid blend mode: %s", str);
 
-		instance->setBlendMode(mode);
+		try
+		{
+			instance->setBlendMode(mode);
+		}
+		catch (love::Exception &e)
+		{
+			return luaL_error(L, e.what());
+		}
 		return 0;
 	}