gl renderer: Set an error if we don't have one already

We can't fail and not set the error, since caller don't expect that.

gsk/gl/gskglrenderer.c

@@ -3035,6 +3035,10 @@ gsk_gl_renderer_create_programs (GskGLRenderer  *self,
 out:
   gsk_gl_shader_builder_finish (&shader_builder);
 
+  if (error && !(*error))
+    g_set_error (error, GDK_GL_ERROR, GDK_GL_ERROR_COMPILATION_FAILED,
+                 "Failed to compile all shader programs"); /* Probably, eh. */
+
   return programs;
 }