Throw 404 instead of an unhandled exception when an asset isnt found

I used to use the "assets" folder for a different purpose and after switching to asset-pipeline, some user-agents are still making requests for files that don't exist anymore. This was triggering hundreds of errors in my inbox because the InvalidPath exception was unhandled

I have modified the AssetPipelineController to return a 404 if the path cannot be found, thereby avoiding this.

I was wondering if the 404 should only be returned in production - what do you think?