Allow defining an error-handler when asset could not be found

[kdevo]

Additionally, we should allow defining a default behavior (error-handler config key) when the specified asset could not be found. Some ideas:

• error-handler/warning.html: partial that just prints a warning when an image could not be found
• error-handler/error.html: partial that prevents site from being rendered when image could not be found (current behavior)
• error-handler/static.html: partial that prints a warning, but tries to use the image statically (from static folder)