Revert "Change naming convension for error templates"

I don't want to type long status names in words...
This reverts commit daed594c24952cf3322d358f289986e47d520de0.
Change template names in favor of 46c7a6b146e28f01470ccb167fb768bd78f984cd
Render 500 page/json if the corresponding template is missing
Update README