Create error pages for common errors

Currently if an error happens only a string is returned to the browser. Create pages for common errors:
- 401 unauthorized
- 404 not found
- 500 internal error

Every time an error occurs render the appropriate error page, or no page is defined, then render the error string. Always set the correct status code.

For images, create a 'not found' image and serve that if the image is not found. The dimensions of the 'not found' image must match the requested image's dimensions. For example trying to access the image at `/97bdbe337edb25031be6ba9fd499ec19cf3ccaad` path, and there is no image with that hash, then a standard-sized error-image has to be sent back. When the width is specified (`/97bdbe337edb25031be6ba9fd499ec19cf3ccaad/500`) the error-image must match that width.

Start by locating the request handlers for the 401, 404, and 500 responses and for image paths with optional width values. Define the required error-page fallback and status-code behavior, then verify that missing images return a not-found image with the requested dimensions.

Written by the indexing model from the issue text.